In terms of the degree of faffing about with gear, I think tiny backmount doubles is the deal for redundancy with modest gas. I got Erick's gorgeous freedom plate and have it mostly configured. Yet I see little chance I will use it much as it would mean carrying a pony to have redundancy. My backmount 40s (or some 50s) are a much easier unit to manage on land and in the water. Though by manage I do mean things like removing and replacing in the water.
I dive sidemount with a bit more faff, but it gives me extra benefits.
The 50s are much easier on my knees.