gfx card hierarchy chart:http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
gtx 550ti (192 bit) is an update of gts 450. nvidia set it to compete against radeon hd 5750/6750 and 5770/6770. it trades blows with 5770/6770. gtx 460 (256 bit) 1 gb is more powerful than the aforementioned cards.
550ti is overpriced for its performance, uses more power than 5770/6770 but delivers almost similar performance. get's outperformed by hd 6790, gtx 460, hd 6850.
only pros would be cheap (for nvidia cards) card that has support for cuda and physx.