You might be able to ninja a much better card for $75 such as a GT 240 or a GT430 instead of a weak GT220. If you are set and stubborn with only those two choices then settle with the 5550 due to raw performance.
It is a SFF case (Dell 755) and a weak PSU so it needs to be low profile and low power draw.
I was leaning towards the 5550 myself due to reviews and specs I read.
I just didnt know much about the GT220.
The 5550 is 35W max draw.
Also like that is DX11.
I am not a heavy gamer.
I like a little L4D2 (1920x1080 med settings) and Crysis Warhead (1280x1024 lowest settings) and Far Cry 2 (1600x1200 med settings) sometimes but I have been playing them with a OCd 7600GS AGP so this will be a big improvement for me.