Yeah, the 660ti is excellent but is about twice the price (and performance) of the 550 ti. The decision for OP is how much money do you want to spend. Also, consider that if you get the 550 ti you will probably upgrade sooner. If you get the 660ti you will be fine for a while with the option of SLI down the track and have great performance all the way. The 2GB VRAM should also help.
In summary, what's your budet?