I'm just wondering whether there are any big(ish) reasons I should get the Canon 600d and not the Canon 550d. I'm interested in the difference in video quality and low light performance (if there is any) and obviously interested in differences in all-round image quality.
The performance of these two is expected to be pretty much identical since they share the same sensor.
If you get the cheaper one (550D) then you should invest the difference towards a better lens. This will have much more impact on your photography than anything else.
EDIT To answer comment. Compared to the 550D, the 600D:
- Lacks an Eye-Start sensor, you have to manually turn on or off the status display.
- New 'intelligent' mode added to Auto & Scene modes. More 'artsy' filters.
- Live-View can shoot at different aspect ratios, basically an automatic crop.
- LCD can be rotated outwards and tilted.
- Can control flashes wirelessly.
I know the answers are quite complete, but I still have something to add.
The main reason I went for the 600D was the flip LCD, because I'm quite small and it makes it easier to take shots of things higher up, I otherwise wouldn't reach (and a wannabe-periscope on urban explorations). It's although useful for taking close ups of things close to the ground without lying on wet grass.
(I don't have enough reputation to post the picture here yet. http://riapi.deviantart.com/gallery/#/d41xs33)
I recently reviewed the changes between the Canon t1i, t2i, and t3i on my blog. There are a few substantial differences between the first entry in this line (the t1i/500D), but there are minor differences between the t2i/550D and the t3i/600D.
The biggest reason, which should encourage some people to choose the t3i is the wireless flash control. The pop-up flash on the Canon t3i/600D can act as a master in a Canon wireless flash set-up. Without the Canon t3i, you'll need to either use a Canon 580 EX speedlite or a Canon ST-E2 wireless transmitter to control your other Canon flashes. That's a hefty investment ($200+ for the transmitter, $400+ for a 580 EX).
It's less than $100 to choose (for a body only) the Canon t3i over the Canon t2i, so anyone that plans on using Canon e-TTL flashes wirelessly should definitely choose the t3i.
A slightly smaller reason is the articulated screen on the Canon t3i. This might be useful for amateur photographers who use the LCD screen to compose photographers. It's more likely to be useful for videographers who would like to film from an awkward position and can't see the back of the camera. I never use the LCD to compose photographs; however, I would definitely like being able to use an articulated screen if I used the video mode on my Canon t1i more often.
If you don't use wireless flash, you don't have a great reason to choose the Canon t3i. And if you don't do video either? Save yourself the money and take the Canon t2i. It's not a huge upgrade, though, so don't expect to save a lot of money.
You might want to check DPReview.com for a detailed hands-on preview and comparison with the 550D: http://www.dpreview.com/previews/canoneos600d/
I really think there is no significant change in image or video quality. Physical changes have occurred, but I wholeheartedly agree with Itai on this one. It is such a good idea to invest in that better lens than deal with a mediocre kit lens. I like Canon's quick upgrade, but I really think the changes are minuscule. Plus, it's the glass (lens) that matters.
I have recently purchased the 600D, upgrading from 350D which I owned for 4 years. The 600D is a great shape and very user friendly, easy to handle. The batteries and cards for the 550D and 600D are the same which shows the similarity between the two bodies and differences with the earlier bodies. The flip LCD protects the screen and makes it better to see the controls as you change them; it also helps when using live view as you can take shots at different angles without the need to look through the view finder. The quality and ISO are great on both of the bodies making them similar and possibly difficult to chose between them.