I'm looking to buy an entry level DSLR camera, because I'm anticipating doing a lot of travel in the next few years. As far as Canons go, I think I've narrowed it down to the SL1 and the T3i.
I'm looking for...
1. Easy to Use and user friendly- I've only ever used a digital point and shoot
2. Would be able to get pictures from somewhat of a distance - i.e. if I'm on a safari, or taking pictures of a landscape while traveling
3. Good to travel with
4. I like the lens kit of 18-135 mm the T3i offers (unless the 18-55mm would be suitable for my needs? I have no idea)
5. I know this is a Canon forum, but convince me why I should go Canon, instead of Nikon D3100?
6. Unless you think there is a better camera that fits my needs?
First off I wouldn't compare the T3i to a Nikon 3100. The Nikon 3200 is a more even comparison. Of course I would buy the Rebel T3i but that is because I prefer Canon products.
I don't believe the Nikon has the articulated screen, maybe it does but I don't think so. If that is important to you.
Actually, all this is up to you. Not "our" or "my" choice. You need to go handle both and see which feels right as the photos each produces are going to be great and very similar.
But my choice for you is the Rebel T3i with the 18-55mm IS II kit lens. Or, you can get it with the 18-135mm IS which is just slightly bigger but still a very good traveling partner.
"... is the 18-55mm a good starting point, or is it worth it to just go for the 18-135mm right away?"
The 18-55mm is going to be slighty smaller so a more friendly traveling companion. The 18-135mm has more reach and going to get some better pictures on the long end. Your choice.
Make sure you go to a true camera store and not a Best Buy or something like it. They don't have a clue.
If a Best Buy is your only option for a look see, just buy the T3i or T5i and the 18-135mm. You will love either and the difference between any of them is slight.
Oh, remember, there are more Rebels sold by a huge margin than any other camera made. There is a reason!
The SL1 has the same sensor as the T3i, but it's designed to be especially compact. Note that there's a trade-off here because when you make it compact, you take away space that used to be occupied by buttons. But the SL1 has a "touch screen" LCD on the back.
The T3i does not have a touch-screen (the T5i does). But some peopple actually prefer a slightly larger body (in fact... some would want a body even slightly bigger than the T3i).
The 18-135mm lens is certainly more versatile. Landscapes typically don't require a long lens... usually it's the opposite. If you need to look around to take in the view, then your camera needs to be able to "look around" to take in the view by using a wide-angle lens. If you are only going to take one lens... then I'd rather have the 18-135.
Both will be easy to use. You can operate a DSLR like a point & shoot (fully automatic) but you'll get better results when you learn to take control and influence the exposure settings. There are times when it's better to have a faster shutter or a slower shutter... and times when it's better to have a wider aperture or narrower aperture. When you shoot in full auto, the camera is just going to try to find a "safe" exposure... but not necessarily the "best" exposure.
The Canon vs. Nikon thing is basically a rat hole. You'll find some ways in which one is better than the other... and then you'll find other features for which the reverse is true.
But if you want to watch a video... here's Scott Kelby (Kelby was a long-time Nikon shooter. He is famous for his training school, his books, and he's also a sports photographer) explaining why he switched from Nikon to Canon (he's now a Canon shooter.) This is posted on YouTube:
Is there a better camera? Of course there is... there's always a "better" camera... they cost more, but if you're budget is unlimited, then the T5i is certainly better than the T3i or SL1... and the 70D is even better yet... and we could keep going. There are some amazing lenses in the lineup as well.
In addition to the points the others have made...
For travel, the SL1 might have an edge, as it's a little smaller and lighter.
However, there's no battery/vertical grip available for the SL1... I shoot a lot of portraits and use grips on all my cameras as it makes them much more comfortable to shoot with in portrait orientation.
I'd suggest to put more thought and money into the lens you choose, than the camera you use it upon. The lens(es) make more of a difference in your images, than the camera ever will. All the current cameras are quite capable, it's just a matter of choosing one with the features you need and learning to use them. But you can't make a wide lens telephoto or vice versa, you can't render shallow depth of field effects with a small aperture lens, etc. Ultimately lenses are more important than the camera they are used upon. And the whole point of a DSLR camera is the ability to change lenses, to adapt the camera for use in different situations and to produce different types of results.
The 18-55 IS STM is a decent little, entry level lens.
The 18-135 IS STM is a nice step up in versatility, though still an entry level lens.
Note that in both cases I specified the "STM" lens. These are the latest, and most feel the best versions of these two lenses. STM refers to the Stepper Motor focus drive system they use, which is quieter and faster than the micro motor drive used in non-STM versions of each lens that you'll find at slightly lower prices.
Personally I'd probably choose the EF-S 15-85 IS USM... It's better built, has better image quality than either of the above, as well as higher performance USM focus drive... And 3mm makes a big difference at the wide end, it's quite noticable the difference between 15mm wide and 18mm wide. 85mm is long enough for most portraiture.
An alternative is to get one of the kit lenses, and add an EF-S 10-22mm USM... Which is even wider, of course, and an excellent lens, too.
You mention "getting photos at a distance" and being "on safari", both of which suggest to me that you will also need a telephoto of some sort. The entry level to that is the EF-S 55-250 IS STM... which is really quite a good lens optically. Better built are any of the EF 70-200mm L-series lenses. There are f2.8 and f4 versions, IS and non-IS versions of those. The 70-200s are larger, pro-grade, real workhorses, and priced accordingly. In between priced there are some decent 70-300mm... the IS USM version is decent, as is the more compact 70-300 DO IS USM, which might be nice for travel. For more serious "safari" wildlife shooting, you might want to look at the EF 100-400mm or similar.
Most of these telephotos are much larger and heavier than the wide angle or mid-range zooms mentioned above.
I can't really compare to the Nikon gear you mention. I haven't used them. I do have some vintage, manual focus Nikon gear and it's good stuff. I agree with the suggestion, it would be best to go into a store where you can handle and study each model you're considering. The ergonomics and control layout of Canon and Nikon differ. I'm sure either can be learned, but one might feel more comfortable than the other to different people.