I was issued an MR NICE Crew Cab and 6500 bag. I have hauled things with both the load sling and the crew cab on my NICE frame and I think the crew cab allows you to haul things a little easier. The side pockets can either lay flat against the inside of the cargo area or you can open it out sideways and then use the side pockets to wrap around whatever you have in the cargo area.
Since you already have the NICE frame a Crew Cab bag is very useful. It is heavy and I have switched to a SG X-curve 5900/7900 for most trips but the MR NICE Crew Cab is bombproof and is a great pack. I used to run Kifaru before I switched to SG and I think the MR NICE Crew Cab will survive abuse that none of my other packs would. The thing is overbuilt but you never have to worry about tearing or ripping something.
I strongly recommend both the OV load lifter extensions and the belt slides, for not much money they improve the NICE frame comfort dramatically.