I'd leave it whole. For more bark, trim off the fat cap and any other external fat. That will give more nice surface area for bark. There is plenty of internal fat on a butt to keep it moist.
Cutting a large hunk of meat in half doesn't reduce the cooking time that much.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)