The 3350P is an Ivy Bridge core that had defects in the IGP section, so Intel disabled that section of the chip (not that it's any loss) and is selling it as an ordinary CPU.
Compared to a 2500K, the 3350P has slightly lower clock rates (3.1 stock, 3.3 turbo for the 3350, 3.3 stock, 3.7 for the 2500). Because the 3350P is an Ivy, it has a higher IPC and there wouldn't be very much difference between them at stock speeds. However, the 2500K is unlocked and can overclock much easier than the 3350P would, making it a better deal if the two are in the same price range.
