I'd personally like to see it mixed up a bit... a cluster of dungeon tiles > a cluster of cave tiles > a cluster of dungeon tiles > etc., etc. Maybe even put one or two caves as joiners in a large dungeon cluster, or vice versa. Yeah... I definitely think mixed would look more visually interesting, and better for anyone who actually plans to play through it. I think it would kinda suck to have to play through a massive swath of dungeon before ever seeing some caves (or vice versa).