Finstar :
id say the 870k is better due to higher theoretical processing power
"Theoretical" is the important modifier there. "Theoretically," they should perform hugely. In the real world, however, extremely mild stuff. That's AMD's marketing game. "3.4ghz, quad core, wow! Intel's cost three times as much!" Suffice it to say, there's a reason AMD's specs are so high and their prices are so low. Because specs just don't translate into real-world performance.
The i3 would be the better purchase.