smoboNeither. The i5 2500k is two generations old, and the i5 3570k is one generation old. They're no longer in production, and their prices are going up. Get it i5 4690k, the current generation i5.
http://www.amazon.com/Intel-Core-i5-4690K-Processor-BX80646I54690K/dp/B00KPRWB9G/
If you have a microcenter you can buy from, getting a package deal of motherboard + i5 4690k would be an awesome value. The best thing here is that they don't sell incompatible package deals, so you don't have to double check everything.
EDIT: If you're buying a motherboard separately, you're looking for a LGA-1150 socket, and most likely Z87 or Z97 chipset. Something with an H chipset won't be good for overclocking, and a gentle overclock is a great way to improve your TF2 performance.
If you don't want to overclock, you can buy a non-K version of the processor and an H-series motherboard that still has an 1150 slot. That'll save you a decent amount of money, but the overclocking potential is gone.
Do you think if I oc the 4690k it'll do the trick?