Easy decision, 5850 all the way :) Here are the reasons:
3D- Nvidia go on and on about this, stereoscopic glasses cost a bomb and they cause headaches and eye fatigue and then you need a compatible monitor which is just as overpriced as a GTX470. If you really want 3D Zalman are releasing a new 3D solution with monitors that, according to reviewers, are alot better since the depth is actually outside the monitor and it uses relatively inexpensive glasses.
PhysX- Has hardly any compatible games and really isn't worth the price premium, unless one of the few compatible games happens to be your favourite
Eyefinity- Way better than 3D
Power consumption and heat- The 5850 uses less power and runs alot cooler than a GTX470
Price: The 5850 is cheaper
Overclocking: An overclocked 5850 will EASILY outperform a stock GTX470 and even kills of a 5870 (if you're willing to go extreme). Heres a link
"Hidden Threat": ATI Radeon HD 5850 Graphics Card Review - X-bit labs
Have fun deciding!
EDIT: Btw, I have a Powercolour 5850 based on ATI's reference model, i can confirm its a decent card, good overclocking headroom with decent temps and great stability