How would they compare?
You do not get more memory for rendering at a higher resolution for having two cards. The 560ti is cooler than two 550ti's in SLI and uses less power (560ti 170W < 2x550ti 232W). I would go with the one 560ti because it's a great card and SLI isn't always properly scaled in games.
Also check out this http://www.hwcompare.com/9686/geforce-gtx-550-ti-vs-geforce-gtx-560-ti/
And if you are looking to get a new graphics card go for the 660ti, because it is only $80-$100 more than a 560ti (Dual Fan Cards)
http://www.hwcompare.com/13163/geforce-gtx-560-ti-vs-geforce-gtx-660-ti/
Would the GTX550 ti in 2 way SLI perform better then a GTX560 ti?
The 560ti would definetly perform better.
Lol, what? I'm almost positive the two GTX 550 TI's would outperform a single GTX 560 TI. Everyone always says stupid shit like, "SLI isn't supported in all games." Yeah, maybe not, but it's supported in the vast majority of games.
but for the price of 2 550Tis you can get a 560Ti 448 core, it will perform better than a 560, its a toss up on 2x550tis and a 560ti
2x550Tis=384 cores, 192bitx2
1x 560Ti=384 cores 256bit
of corse there are things like ROPs, "not 100% SLI scaling" and memory bandwidth to consider
If a 560Ti 448 doesn't cost too much more, grab it, if not then I would grab a 560 Ti anyway, less power consumption for about the same performance
I have 2xGTX 550ti in SLi and I also happen to have a GTX 560 (not 560ti). Which would be better to run, in your opinion? I might just bench mark them and see :D