Graphics cards and black friday/cyber monday

I've been containing myself from buying many components for a new build until this week in case of any sales.  I've been eyeing the HIS Radeon 7870 which is now on sale on newegg for $189.99.  However, through craigslist I have found someone willing to sell me a new GTX 670 (this one actually for $200.  I know it retails for much higher but i don't really know why.  I'm feeling that I'm going for the craigslist deal but I was wondering if anyone could tell me the big differences in these cards.