I would pick the GTX650ti.
It is slightly stronger, and with 28nm construction, it stands to run cooler and therefore quieter than the gtx550ti.
Do not anguish over the choice, just buy the best card your psu and wallet will allow.
Modern 28nm cards(GTX6xx and amd 7xxx) are competitive on a price/performance basis.
You will get fair value at any price point, and differences in performance will not be detectable to you.
If you exclusively play only one game, then check benchmarks for that game.
You may find that either amd or nvidia plays better.
Otherwise, if you play many games, ignore the benchmarks.