It just depends.

What terrain will you be riding, and what can you afford?

When I rode out in Boulder, Colorado on the MTB trails (as opposed to the bike trails) the vast majority of the riders had full suspension. If you're really into it, you'll also want disc brakes.

If you'll be riding MUPs and crushed gravel trails, it doesn't make much difference.

If from time to time you'll bike off your roof just for fun, make sure you have great suspension.

If you're older and can afford it, get full suspension for your back's sake.

Everything else is bling for the most part.