both are excellent dslr cameras.
At the entry level you won't find anything different in terms of image quality.
Both 550D and D5100 are hugely popular in the entry level market.
Decide according to your needs and requirement.
I personally have 550D and I love it.
I got a very good deal so I lean towards 550D.
Also I loved their 70-200 F4 which one of my friend has it, so I went with Canon.
So decide by yourself.