The 2550k is a viable option, as is the 2500k, but with the 3570k being Ivy Bridge, the clock for clock performance is around 7% better.
The 2550k has no integrated GPU, which may or may not be a problem depending on if you use integrated GPU's, which for gaming is not recommended.
Though the 3570k would probably be a better option regardless, though the Sandy Bridge will OC slightly better in some cases, and maintain lower temps.