A pork butt roast is usually a pork shoulder anyway. I suggest talking to a butcher at the store (if they actually have one. If they don't, find another store. They're trucking in meat that who knows whats in it.) and asking him the difference in the cuts.
I smoked a blade last weekend, just picked it arbitrarily. Turned out great, and pulled just fine. Fell right off the bone which I burned in the bonfire. I followed the stickey on pork butts in the pork forum. Others on the site have more experience than I so take in their advice also.
Hey Bear... Try saving those bones. And the raw bones from de-boning meat. Freeze 'em and when ya get a couple pounds, first roast them... the raw ones.. for a couple hours. Then simmer them in enough water to cover with a bit of salt and an onion.
Makes great stock. I use it in alot of things from my BBQ sauce to spaghetti sauce.