Auto level means the unit levels itself instead of having to adjust it manually. Turn it on and it does the rest, once it starts spinning, it's level. Manual or auto level will sustain level as long as they are undisturbed. Every auto level laser I've worked with will shut down once they aren't level, say if someone bumps it, so you're not unknowingly building grade off a bad shot. Auto level is a feature you want, definitely worth the extra coin.
Go hard, go fast, or go home