I think the first play session is incredibly important to get someone roped in to the RPG. When I introduced Final Fantasy to Sam, I picked X, primarily for the reasons above. If someone doesn't really 'get into' the game after the first part at least, then they may not bother to play it again. The good thing with X is that it doesn't have a slow start, besides perhaps the bit before the Sin attack in Zanarkand. If I showed FFXII to him instead, a good chunk of the beginning is build-up, which as a fan I can understand, but Sam may find it slow.

Having said that, now that he's used to Final Fantasy somewhat, he may find the slow start less taxing.