The 5870, purely for the fact that it is a single-GPU card and is as-such less dependent on driver support for good performance in games compared to the GTX 295, which is basically two GTX 260s SLI-ed on one PCB. SLI support is usually close to non-existent in titles when they release, and is only improved after a patch or two, or new drivers.
I personally hate playing the waiting game, so I'd go for a single-GPU solution that guarantees that I can use all the pixel-processing horsepower, rather than having some of it go to waste, as in the case of dual-GPU solutions like the GTX 295.
Power consumption and heat output are two other concerns; the 5870 wins out in that too.