What are you going for? If you're going for the nostalgia thing keep the pontiac. You don't see to many out there these days. I think nostalgia is pretty cool myself. (A Buick nail head with webbers in a wood deck miller would be bitchin.)
But if you're looking to hull ass it's hard to beat the big block chevy. You could toss a 454 crank in it the next rebuild and have a nice 468". Parts for the BBC are easy to find too.
But then again a stout small block would be fun. But if I was to go through the hassle of changing everything I would go BBC.