Double, because the crownpiece breaks or wears out I can just slide another one on there without needing a halter repair.
It also gives a wee bit more wiggle room on sizing for those pesky emergencies, like, "Hmm, look, a loose pony in the road. All I have is this horse-sized halter. I'll just put it on the top holes on each side, which will make the halter fit and will make the pony look like Eeyore."
Thanks for the clarification I prefer the double buckle for everyday halters and single buckle for show halters. The double buckle is more adjustable, and when my horses' inevitably break the crown piece, I can salvage the halter by punching more holes or buying a new crown piece. If you break a single buckle crown, you have to buy a new halter. However, I do think the single buckle is more attractive, so for shows I prefer a thick leather (less chance of breaking) single buckle crown.