I think the series is in serious need of a game engine update. If I had to choose, I'd pick Source simply because of the free Source mods that accompany it.
cs 1.6! is a classic. i've played both 1.6 and source. both great games. source has nicer graphics whereas some people like 1.6 due to things like walling is easier generally, guns are more "realistic" with recoil etc. i still play 1.6 pm to play me