Both the USP-S and P2000 are solid starting pistols for Counter-Terrorists in CS2, but they serve slightly different playstyles. The USP-S comes with a silencer, better first-shot accuracy, and is excellent for players who prefer long-range precision and stealth. The P2000, on the other hand, offers a slightly larger magazine and is better suited for close-range fights.
No, you cannot swap between the USP-S and P2000 once a match has begun. You choose your default CT pistol in your loadout settings before entering a match, and that choice stays locked for the duration of the game.