I've been looking at getting these also but haven't decided yet either. XFX has good sales on them and you can get a 7870 for a good price from them, maybe even around a 7850 depending on the brand you get.
I do feel the jump in price is worth it if you look at the difference in stats.
it depends how much you value you $40 but heres a good comparison so you can see exactly the different performances http://www.anandtech.com/bench/Product/549?vs=548
I will say though viktanoob is right about the overclocking so take that into consideration
Hm that really opens my eyes to what I thought would be worth it..
The differences aren't major, 10 fps being the most I think in a FPS difference. The only thing about OC'ing is it voids the warranty, so if it gets messed up by your OC or something random by chance you will have to fork up the money all over again, so in that sense spending the extra 40 bucks might be better in your eyes. I know that part speaks to me because I'd hate for something to go wrong down the line and have to replace it just because I OC'd.
I've been trying to weight the pros and cons for about a month now and its been a hard decision.
I should have said as well don't go for the visiontek one purely because it has a single fan it will be loud at high temperatures there is a few twin fan ones on Newegg that I would go for before the visiontek one
Safe or not, it voids the warranty. So if the card does something completely different that isn't related to your OC, you are still forced to buy a whole new card. This is something to take into consideration.
I bought a 7850 1GB GDDR5 version and my friend just recently bought a 7870 1GB GDDR5 version we tested on the same rig (8350 @ 4.5GHz, 16GB DDR3-1866MHz RAM, 120GB SSD, etc) and their was no clear difference noticeable to the human eye between the two. In more graphically demanding games such as Crysis 2/3 and Battlefield, the framerate was higher but not high enough to justify the $40 extra.