I enjoy having more VRAM
The GTX285 also has higher GPU and memory clocks. It's honestly a toss up. All depends what you are looking for / willing to spend / what games you play / what settings & resolutions you plan to use.
The GTX285 as a single card is by far the most powerfull single card solution on the market, the GTX295 will score higher in FPS (except in a few) and benchmarks but keep in mind its really 2 cards doing the job to get it there. From what I have seen anyway depending on the game the difference can be minimal or decent but both cards blow anything out of the water thats out now and neither card slips into the "unplayable" category of any game out right now