I have no less then 5 bags right now and use all of them for different applications depending on what I am doing.
Currently I have the following;
Lowpro Slingshot300
Lowpro Specialist 85 AW
Lowpro TLZ AW (older version of the top-loader 75 AW)
Lowpro EX-120
Lowpro Rezzo 110 AW
The first two will hold pretty much everything in my sig on their own though the Slingshot300 is a tiny bit roomier then the Specialist...but the Specialist is easier for me to manage on my shoulder and has enough space inside of it to have my 5D gripped, with any of my primes with hoods mounted, ready to shoot in an instant and still leave room for a couple of additional lenses.
The other 3 I'll use for smaller adventures around town, including putting the 50mm on the 5D alone and fitting that into the EX-120 or using the TLZ with the 5D gripped or not with a spare lens on the side.
Cheers! 
