Depends on the games. If the games supports SLI very well, it's likely 550 ti SLI's going to be better. For others, 480 is going to be better. But still I will avoid 480, except these two is actually the two cards you can get, There is still better card than 480 like 6970, 7950, 570, etc. If you have the 550 ti already with you, it's better to SLI it and save money.
Iv seen various benchmarks for the 550 ti vs. the 480 and it just comes down to your situation. They both cost about the same to set up but if you have a 550 ti already just by another because there so close in performance you would just be wasting a 100 bucks. But if you don't you should go with the 480 because single card set ups have fewer errors, also you have the room to upgrade and get more power