The GTX295 performs marginally better, but the 5870 is better overall.
5870 is a single GPU instead of dual-GPU 295, so it scales better in games. Also, putting another one for Xfire scales MUCH better than quad-GTX295
5870 also uses far less power, and runs much cooler than the GTX295 (5870 only needs 2x 6pins, the 295 needs 1x6 pin and 1x8 pin)
5870 supports dx11 and eyefinity.
Get the 5870!