As far as a gaming CPU, Tom's ranks both in the same tier: http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106-5.html
But if cost isn't a major concern, I would go for it. O/C the 3870K to the max. The 560ti should give as good a result with that chip as it would with the others in the upper half of that tier. None of which are exactly slouches in gaming.