So going into purchasing sacs i considered both created equal. There have recently been quite a few posts on issues with getting the sumo pump to prime... and while it does take 2 people to get it to work right i find this a small price to pay to get the one way check valve. I ride with a buddy who uses Fat sacs and we both use pumps thrown over the side to fill. So if you are going to be manually filling sacs i think there really is no equal to the new sumo sacs with the one way valve.
1. you can fill them completely full and dont have to worry about water spilling out
2. when you empty them you dont have to worry about quickly connecting the pump b/c the one valve is there to prevent water from shooting out.
Just my .02 for what its worth ... for anyone out there thinking about which to go with.
Note: one plus side of Fat Sac is they have been around longer so they are easier to come by on eBay, and forum classifieds.