PvP is largely an optional thing for you to do in either situations. The first one sorta allows it since there's no inheritance reason why any of you should work together (though it's highly advised you do for your own survival) whereas the second one is expected since you're all going to be gathered as a team.

As for the campaign length, I'm in no rush. Frankly I don't expect to even [i]finish[/i] the campaign due to the potential length, at least for the second plot idea. Again with the first plot, that's largely dependent on the players own desire to continue to delve into the dungeon world.