I took your original question to mean that you wanted opinions on loin OR butt; in that case I'd go with butt.
If you want an additional meat with the butt and you want pork I'd go with tenderloins. They brine/marinate well, you can brown them quickly then high-heat roast to 140, cool, wrap in foil, and reheat (at 225, say) to 145--very manageable. While it's possible to make loin work at 225 (brining of course and/or injection, and using your suggestion above of browning first), success is less assured using this approach, imo, than using the better-suited higher heat approach. That said, were I to do a loin I'd select one carefully, brine 24 hours, plan to do it day of to avoid the reheating issue, pull at 145 tops, rest 20 min, and slice and serve in one fell swoop.
Though 225 is low in my book so, obviously the meat cooks slower, I tend to think of 'low and slow' as also meaning taking the meat to its well-done stage, which you can't do with loin. Just a difference in semantics, I meant no disrespect.
