You are asking this question in a JBoss forum? Why, JBoss is the best, of course! ;-)
But seriously, a little googling should return some comparisons, some even by people who have actually used both products to build system. But be careful with opinions from 3 or more years ago - both app servers have changed since then. And JBoss AS 7 is radically different from prior versions; I doubt you'll find any posts that compare it to other app servers.
Finally, why not do a prototype on both app servers? See which one works best for you and your team. After all, if you are betting your business on this, why wouldn't you want to perform such an evaluation. But if this is just for personal use or a school project, then JBoss AS would be the best bet simply because of the cost.