The option was not "any serve you like," but "Sampras' serve." As such, why don't we quit bringing Karlovic's serve into the equation? Otherwise, in point of fact, some might be interested in having, say, a peak Fernando Gonzalez's forehand instead of Federer's-- but again, that isn't the question.
Of the two strokes, I would certainly prefer Sampras' serve, because the serve is a more important stroke than the forehand; if you had no other major assets to back either stroke up, you could still win quite a few matches on the strength of that serve, while the forehand wouldn't do you nearly so much good.
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