GTX 275 runs cooler, needs less power, newer and was just released 2 months ago so it won't be discontinued for at least 4-6 months, which means that it will be better if you are thinking about going SLI down the road.
GTX 280 was released 12 months ago and is being phased out in most etailers meaning it will be even harder to find later if you want to SLI.
Overall GTX 275 is a way better choice than the 280 and performs roughly the same.
GTX 275 produces less heat than the 280 therefore allows for higher overclocks but has a 448-bit wide memory bus vs. the 280's 512-bit wide bus which means less memory bandwidth (plus the obvious 896mb vs. 1GB of Vram difference)