the M7 bayonet is superior in durability to the M9.
the M9 isnt even really a true bayonet at all, but more of a survival knife that they added bayonet functionality to. if you do some research on them the net is rife with stories of servicemen breaking them very easily during simple training drills. whats funny is that despite the fact that the M9 replaced the M7, ive read that in many cases they switched back to older M7s for bayonet training drills because of the high failure rate on the M9s. the M9s have a very poor reputation among all but the mall ninja crowd.
if you insist on the M9 style, then perhaps check out the new Ontario Marine bayonet - OKC3S - its supposedly a more durable, improved M9. might be something worth looking into, but theyre more expensive.
if i wanted to stick anything more resilient than a cabbage, however, i would stick with the M7 - its basically a thick metal spike, and very durable and proven. they can also be had cheap on ebay - especially if you dont care about collectability.
it may not look as cool as an M9, but IMHO function/durability vastly supercede looks - and they have an aesthetic appeal of their own. if someone wanted to give me an M9 for free id take it, but i would not pay a penny more for a Mossberg that came bundled with an M9. id just pick up an inexpensive M7 from ebay.