I would say a compound. Reason being that a compound can shoot a heavier, longer, more stable arrow, and you can reload faster if the need arises. The longer arrow means you can take a little longer shot, have more kenetics to punch through the thick hide, and have more likely pass throughs.
I'm in complete agreement with hunter9022. However, I doubt an enraged, wounded boar will give you enough time to reload either weapon before you're chased up the handiest tree.
Nor would that hog give you much of a chance of finding it in a crossbow's scope. A conventional bow is much faster into action in that kind of scenario than a crossbow.
The idea is to shoot while they're fairly quartered away. That puts the arrow going in behind the gristle plate for easier penetration into the vitals. Plus when pigs are hit, there is a much better chance of them running in the direction they are facing. Namely, AWAY from you.