Here is a comparison of the 650 and the gtx 285http://www.anandtech.com/bench/Product/681?vs=520
judging by that the 650 and 275 should be approximately similar in performance.
So I would use the gtx 650 as it is newer, has MUCH lower power consumption and will be cooler and quieter. Also the 275 will not be able to use direct x11.
Edit... I also agree with Cons29... you have them both, try them out and see which is best