In theory, 5/16 should be better at preventing leaks, all else being equal, given that 7.93mm is a tighter fit than 8mm.
I believe that the BMW hose is less likely to leak due to it's construction. I will be going to the BMW dealer this week to replace the 5/16" that I installed yesterday.
(For those of you that don't own one of these beasts, we're not beating a dead horse. A fine spray of leaking fuel in an engine compartment noted for it's high heat and tight confines is a recipe for disaster.)