As a former T1i owner, I can tell you that the T2i was a big jump. The resolution improved slightly, but mostly the ISO performance improved. But with the T3i, T4i, and T5i the difference in sensor performance has been negligible. One would probably not actually "notice" the difference without doing careful scientific analysis.
The biggest differences between the T2i, T3i, T4i, and T5i are all body features -- not differences in image. The T4i introduced a focus system which looks just like the focus system on your T1i... except that on your T1i only the center AF point is a "cross type" point (performs phase-detect focus in two axes simultaneously) and the remaining 8 points are single axis only. But on the T4i (and T5i) _all_ 9 AF points are cross-type points. These are more accurate and are difficult to trick into missing the focus. Also the LCD screen on the back of the T4i and T5i is a touch-screen.
So if you "broke" your T1i and needed to repace it, I'd just get a T5i.
But the key here is that you haven't broken your T1i. You're looking to upgrade. And apart from the ISO performance boost, you wont notice much else.
The 7D is getting old in the tooth and due for replacement. Rumors speculate but the latest rumors suspect it's probably still a year away (those who actually "know" aren't talking.)
The 70D, on the other hand, is quite a jump. It has all the ISO performance of the rest of the cameras, it's a mid-level body (the Rebel series are entry-level bodies), and it's only 1 single frame per second slower than the 7D when shooting in continuous burst mode (for sports). But it does have this new dual-pixel AF on the sensor which allows the camera to perofmr on-sensor auto-focus continuously while shooting video and it's much more accurate than the contrast detection auto-focus systems of previous generations. For video... it's a huge step up.
As a mid-level body, it gets a degree of weather-sealing (not weather-proof and you should definitely not submerge it in water) and it has two control dials (rebel bodies only have one). The second dial (as with all "pro" bodies) is on the back positioned nicely for use by your thumb while the front dial is near the shutter positioned nice for your shutter-finger. This allows you to adjust both shutter and aperture silmultaneously when shooting manual and when shooting in any of the semi-auto modes (Av, Tv, or P) the second dial serves as an exposure compensation dial.
It also has a top-LCD screen which several extra buttons along the display for instant access to commonly used functions that a Rebel body normally requires longer to set.
The biggest dramatic difference, however, is the 6D (and up... such as the 5D III and 1D X but those are extremely expensive bodies). The 6D has a massive boost in ISO performance. You can shoot, for example, at ISO 6400 and not really worry about it. On a Rebel body I try to keep it ISO 800 or lower and begrudingly move up to ISO 1600... but pretty much refuse to go any higher.) Sure you'll have some noise at ISO 6400... but it's actually fairly low.
Both the 6D and 70D are WiFi equipped (and the 6D is also GPS equipped) and can be remotely controlled (Canon offers a free smart-phone app).
Tim Campbell
5D III, 5D IV, 60Da
