I have this question too but I will mainly play at 1680 x 1050? Is it worth the 50-80$ difference?
performance-wise, the 7850 will do fine at that res, and offers the best bang for your buck at any res below 1080p.
assuming you're not going to upgrade that monitor to a 1080p, the $50-80 question is; how long are you expecting it to last? games are just going to get more and more demanding, and it all comes down to your expectations. if you're the type who can deal with compromises surprising well, lowering graphics settings as your GPU ages, then it's not unrealistic to hang on to that 7850 for 3-4 years. while my 6870 is showing it's age, based on what i see of FC3 & Crysis3 @ medium-high on YT, i'm comfortable sticking with it if buying a new GPU anytime within this year is not an option .
but if you're the type who can't stand to play at medium, and must play at >high for as long as it takes, a 7870 kinda gives you a little bit of future proofing. maybe not a whole lot, but it's there. personally, if you can save up $10-20 for the next two years though, an extra $50-80 now for a little bit of future proofing is just not worth it.
again, this is assuming you're not upgrading to a 1080p monitor; otherwise, the 7870 gets my vote. a 7870XT/LE is a definite must if that's still in stock.