which one should I pick
which one is better for rendering?
Although 660ti should be a bit more powerful than the 7870, I believe that FX6300 will bottleneck the 660ti in some games compared to an i5 3570, 3350p or w/e.
Anandtech: Intel Core i5 3570K - 3.4GHz vs AMD FX-6300 - 3.5GHz So in my opinion you should go for i5 + 7870.
edit: the i5 is also better for rendering as you can see on the rendering benchmarks.