Just to poke the bear, I'll toss out the thought that the very easiest bag to get out of the way for a fill up or to move away from the horn button is the magnetic kind. The one I have now is from Roadgear, but there are many makers and styles. What I think is important are getting the size you want (with an all-around zipper for expansion, and a map holder that's big enough to slide a decent sized map into without so much folding you ruin every map the first trip out. Number of pockets, rain covers, little holes for wiring connections, etc. are all personal choice things. Mine's pretty big but not big enough to be in the way (and easy to move back if it's hitting something). I had one big bag that hit my kill switch in a sharp turn one time--took a couple of frantic moments to figure out what the hell happened.
Some worry about possible scratches from the magnets. I've got no visible scratches from mine, but a nice assortment from various other hazards. Of course, I don't wash my bike either.
One more thing--if somebody wants to steal your bag, or what's inside it, easy, no matter what you use. But if you want to bring it in overnight, another point for the magnets.
