Snowball helps your team, Devour helps you. With the last changes to snowball, it's amazing to max because it's essentially a permaslow at rank 5. No one can simply get away without plants or strong mobility with your bloodboil/ice blast assault. I personally find this the better option, and opens up your build to a possible early AP item cheese build, which can turn some lanes around.
Maxing devour would be just for counter jungling and objectives. Very easy to run into the enemy jungle and smite/devour a big monster, and just walk out. The enemy should also in general never see another scuttle crab again. Do this when you get pretty stoned and want to play an amusing single-player Nunu.
Actually, maxing Q has one big advantage. If you counter jungle well enough, you'll force the other jungler to gank since he'll be gold starved. Ganks are a lot less effective when they're expected. Just be sure that if you're counter jungling, try to do it when the blast plants are there since that'll be your anti-collapse in case of wards.