it is cheaper to fill a 10 pounder as opposed to a 5 pounder twice, plus the hassle factor as MarksReef points out...if you have a nearby beer brewing or gas supply house, that will help...if it is 30 minutes away, that's another story.
Initial purchase price isn't a lot more. The only real disadvantage to the 10 pounder is that it takes up more room which may be in short supply.