Hard to estimate how much weight one pack or the other will carry but suffice it to say that these Mystery Ranch packs can saitisfactorily handle in excess of 100 lbs loads and do so routinely in military applications.
You don't need any accessories with the Crew Cab although they do sell Load Cells which are not a bad idea. Some Granite Gear waterproof stuff sacks will also work.
The two best expandable packs on the market that also serve as very good day packs are the already mentioned Crew Cab from Mystery Ranch and the Eberlestock J34 Just One. Two different ways to do the same thing. My personal preferance at this point based mostly on the frame is the Crew Cab but you may see it differently.
Can't help you with price, I don't pay much attention to that. Sorry.
I suggest buying both, load them up and wearing them around the house. Keep the one you like best.