Others with much more experience will soon chime in.
I was in the same dilemma except I considered my budget limited to the 1100D and the Nikon D3100. After considerable research I decided to expand my budget and I purchased the 550D.
My reasons:
.1080p video at 30 fps and 720p video at 60 fps! I wanted this for both video and still photography and a lot of my video will be of action and sports. Capturing in 60 fps and then slowing it down afterwards is awesome.
.The screen on the back is larger and much higher resolution. This is my first DSLR, and while I spend all my time photographing through the viewfinder, reviewing images on the back screen is still a priority for me. Helps with the video as well.
.Higher IQ (albeit, not a huge difference)
.Self-cleaning sensor. Again, no expert, but this sounds like a huge plus for durability and longevity.
.Higher megapixels (while this is debatable, it looks nice on a spec sheet)
.Less noise at high ISO
.Finally, just felt better in my hand. Build quality felt better, materials felt better, form factor felt better.
I didn't see the point in spending more for the T3i in my situation. However, the jump from the 1100D to the 550D seemed important and, after a week of shooting with this camera, I am still very confident in my decision.
Good luck and let us know what you go with!