Your problems are probably due to a combination of factors. IMO, auto focus performance is determined by three things....
1. Camera AF capabilities. Your 600D has a 9-point AF system. Only one of those points - the center one - is the "better" cross type that's more responsive and tracks movement better. When shooting moving subjects with your camera, it would help to limit yourself to using only the center point. (Note: other Canon cameras have more advanced AF systems. The T4i, T5i, 60D, 50D and 40D all have 9-point AF that looks similar, but all nine points are the "better" cross type sesnors. The 7D and 70D have 19-point AF, with all nineteen cross type. 1DX and 5DIII have 61-point AF, with up to 41 cross type, depending upon the lens used.)
Some cameras are also optimized for focus performance. For example, the 1D series models and 7D have a separate chip driving AF.... while most (maybe all?) other models share AF duties through the same processor that's handling images.
2. Lens auto focus capabilities: Yes, a "USM" (Ultrasonic Motor) drive lens such as the EF-S 17-55/2.8 would be an improvement over your kit lens. It's faster, hunts less and is quieter than a "micro motor" drive such as is used in the less expensive kit lens. (Note: there is a slightly more expensive version of the 18-55mm with "STM" or "Stepper Motor" focus drive, which is better than the micro motor version, but still not as fast as USM. STM lenses are quiet operating and particularly well matched for video.)
A larger aperture lens, such as the 17-55/2.8 (as opposed to your 18-55/3.5-5.6), also delivers more light to the camera's AF sensors, to allow for faster focus and better tracking.
No, forget about the EF 50/1.8.... Sure it's got a larger aperture, but it uses a micro motor and is widely known to be slower focusing, less accurate and liable to hunt more than a USM lens. If you wanted a fast and sure focusing short to moderate telephoto, look at the EF 50/1.4, 85/1.8, 100/2 or 135/2L lenses. These all use USM drive and are much faster and more accurate than the EF 50/1.8. They also have one to two stop larger max aperture than even the 17-55/2.8 (or any other zoom lens), so provide more light for the camera to work with. Using a lens f2.8 or "faster" will give optimal performance on your camera, with that center AF point.
Some lenses simply are not designed to be fast focusing, even though they have larger apertures and USM focus. Macro lenses, for example, have to move their focus group a long, long way to go from infinity to 1:1 magnification so tend to be slower. Also, very large aperture lenses such as the 85/1.2L typically are slower.... by design. Both macro and very large aperture lenses emphasize precision, to deal with shallow depth of field effects, rather than speed.
Other lenses are designed for very fast focus. All the 70-200s, the 100-400, and all the prime lenses 300mm and longer are quite fast focusing. Extreme telephotos such as 500 and 600mm can be hard to get and keep on target, though.
Some third party lenses also make use of focus drive similar to Canon's USM, to help with focus performance. Sigma's HSM and Tamron's USD lenses are examples.
3. User skill is the third key factor effecting focus perfromance.... i.e., your technique.
You mention using "AF Servo"... Just to clarify, yourcamera has three focus modes: One Shot, AI Focus and AI Servo... there is no "AF Servo". The correct mode to use with moving subjects is AI Servo. Maybe that's what you are doing already. At any rate, One Shot is pretty much only usable with stationary subjects (there are "pre-focus" techniques where it can be used with moving subjects, but those are fairly uncommonly used). AI Focus isn't really a foucs mode at all... It's supposed to decide for you whether or not the subject is moving, then switch to use the correct mode. I haven't tested this on any recent model, but older ones where I tried it I found a slight delay that causes a lot of missed focus shots. It also simply chose the wrong mode sometimes. It might be worth noticing that the more pro-oriented Canon models don't even have AI Focus... they only offer One Shot or AI Servo.
So, just be sure you are using AI Servo for the best performance when shooting moving subjects.
You also are using way too slow a shutter speed. I would bump up the ISO and try to use 1/500 or faster shutter speeds. 1/320 or 1/400 is very marginal, trying to stop subject movement with something as quick as dogs in action. In fact, the closer you are to the subject, the faster shutter speed you will need to truly freeze movement.
Canon also has noted that the 18MP models are somewhat prone to camera shake blur - probably due to the high density of pixel sites on the sensor - and recommends keeping shutter speeds up to be sure to get a sharp shot, even with IS lenses. There was a white paper about this, on Canon's website. I don't know if it's still available online.
You should be able to get quite nice, clean images at ISO 1600... a full two stop higher and allowing you to use much faster shutter speed. Even so, if shooting indoors there might be areas that aren't as well lit and are hard to get a sharp shot.
You may want to shoot RAW files, so that you can more fully control noise reduction and exposure factors in post-processing when using high ISOs.
A popular technique among sports/action photographers is Back Button Focusing. See this article online, about it: http://www.learn.usa.canon.com/resources/articles/2011/backbutton_af_article.shtml
BBF separates the focusing function from the shutter release button. You instead use your thumb on one of the camera's rear buttons to start and stop focusing. Many users feel this allows more sure acquistion and tracking with moving subjects. You may want to set up your camera to do BBF and give it a try. While it's particularly possible for action/sports/AI Servo shooting, it's actually usable with One Shot, too... I have used it pretty exclusively for some years now.
Using BBF, I start AF well before I want to take the shot, then concentrate on keeping the AF point on the subject right where I want the lens to focus while continuing to track and maintain focus... then take shots along the way. I use the center AF point alone, much of the time. This does lead to overly centered images, so I try to frame a little loosely some of the time, allowing for some cropping to make images less centered. I do sometimes use other than the center AF point... but wouldn't recommend that with action shooting on a camera that doesn't have cross type sensors at the peripheral points, such as your 600D.
I use a pair of 7Ds and a number of fast focusing USM lenses to shoot a lot of sports. With some years of practice under my belt, on avearage I nail focus on 95 to 98 out of 100 shots using this gear with the above techniques. I shot 4000 images at a recent event and marked just over 30 of those as "rejected" for missed focus problems (and I bet at least half of those are my fault... not the gear's). There may be a few more that are marginally acceptible (i.e., can make a nice 8x10 print, but won't look good any larger than that). Still, even if there are 80 or 120 missed focus, that's only 2 or 3%... a lot better than only 10% in focus.
I would say that lens performance qualities and user techniques generally count more than the camera though, or at least can largely make up for any camera short-comings. I got nearly as good results with 50D, and with 30D before that (which have similar AF system to your 600D), and even with 10D earlier than that (a "lesser" AF system than yours).Though I've gotten a few, I haven't had as good luck shooting moving subjects using 5D Mark II (I mostly just use it for stationary subjects).. the 5D Mark III has a much improved AF system and tracks movement far better.
Hope this helps!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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