Andy Roddick probably had the best 2nd serve of his generation.
Between the two of them Pete Sampras was definitely more prone to double faulting. (As Roddick would frequently finish matches with fewer than 2 of them.)
However, Pete also used his 2nd serve differently and could even win games against top players with just the second serve alone.
In my opinion, what truly separate Sampras from a player like Roddick was Pete's ability to volley.
This match really illustrates the differences in their games: