6-9 on the bench is what I prefer, even with an evil 30 round mag. A buddy of mine used the 9-13 and did need a lot of rear bag, has since switched to a 6-9. I use it prone also, at least for my bolt guns.
From what I can tell it really depends on how tall you are. I can shoot the 6-9 just fine off a bench but my brother says its very awkward to shoot. I stand in at 6ft and he is 6'6".
I have a Harris 6-9 for prone and bench matches and have not seen the need for anything taller expect for one stage at a match. But to get past that we just threw the rifles up on a backpack.
Remember it's always easier to add than it is to take away.