I use both. Stuff I need to access while riding goes in the pommel bag (drinks, snacks, chapstick). Emergency kit stuff, spare hoof boots, lunch/horse snacks, goes in the cantle bag.
I like heavy stuff (water bottles) up front so the horse doesn't have anything heavy behind the saddle in the weak area near the loin. Also it is easier for me to get water bottles out of the front bags, I can't get into the back bags while trotting. However, the pommel bags sometimes get the excess loop of rein stuck on the bottom edge if I'm having a tight reins kind of ride and have a lot of excess loop hanging ;) That isn't as bad with the snugpax as with other brands I've tried.