According to Anandtech.com, GTX 470 is between 2% to 8% faster than ATI HD 5850. If the directx tesselation matters to you that much, go with GTX 470. 2 months back, at 280 dollars, I thought HIS HD 5850 iCooler V (plain) was a better deal than 340 dollars on a GTX 470 because then at 340 dollars you are really close to HD 5870 which seemed to perform better in benchmarks than GTX 480 (and way better than GTX 470). But of course you get more memory with GTX 470 than HD 5850.
While PhyX on nVidia cards can be useful in games, I saw some tesselation demos, and I could live without it even at the highest resolution with all settings set high. You know, PC gaming these days is really what we make of it
On AMD platform, I would go with ATI chipset. I am using my ATI card on an intel based platform because I thought Phenoms were very power hungry when I built my computer. Talking about power, ATI card (especially the HIS branded ones run well under 65 degrees under full load, thus will last longer). Just a pointer.
Good luck.