Which camera bag?

[James_EG]

Hi, I have been looking for a new camera bag for a while, I want a backpack as my Crumpler Cupcake 7500 gets very heavy on my shoulder with a 60D + grip and just 2 lenses, I have looked at the Vanguard UP-Rise 48 and Booq python Pack, they look almost perfect, but I cannot decide on which one to get, as I am not sure that they could accomadate all my equipment in the future, as I want to buy a few lenses (about 4-5) a tripod, flash and various accessories like filters etc., and the telephoto lens I want hasn't even been released yet (Canon 100-400mm f/4-5.6 L IS USM) so I am not sure on its size. Also I would prefer it if the bag had one of those nice side access pockets so I can get to the camera and a couple of lenses without taking it off my back.

Thanks, James