I have the heavier black 10L for water storage and you DO NOT NEED THAT for a bladder. Go with the drom lite. The heavier one has 1000D cordura on the outside and you could drag it behind your truck going down the freeway and not get a hole in that thing.
I was going to get the 4L Dromelite for hydration and the 6L Dromedary for my pack for rough-use, etc. Does that sound about right?