7950. Reasons:
- Much Lower power consumption during idle and full load
- Scales really well with crossfire
- Can do eyefinity (3 monitor set up)
- Less heat output by around 10c
- Overclockable to almost 7970 performance
- Generally equal performance to the GTX 580, slightly better or worse depending on the game
- Uses 6+6 PCI Express pins compared to 8+6 on the GTX 580
- 3GB VRAM, so you can run high resolution textures to your heart's content without every worrying about not having enough VRAM (and is great for eyefinity)
- PCI-E 3.0 for use with Ivy Bridge
And yes I'm a GTX 580 owner myself. There are still actually reasons I'd rather go with a GTX 580, however it's pointless because Kepler is around the corner and that next GPU will most likely be more worth it. But if you absolutely cannot wait now, then my vote goes to the 7950 still.