Both Iveco and Merc offer chassis that support 7-8 tonnes and require a LR/MR licence. Actual payload is going to depend on layout and fitout materials of course. That seems to be an issue with many builds whether homebuilt or low end professional, they often use heavier material because it cheaper. There does seem to be a lot more Merc based Moho and campervans is the used market and if that's your arena then that could be the decider. btw New builds stretch to 12 or more due to base vehicle availability. My neighbour has a Paradise Iveco Moho and while it has higher gvm and payload than the Trakka Merc camper we are considering, most of the extra capacity is longer term not everyday storage. That might suit your needs.

Both are represented in the hire fleets.
Try each out for a week. Small bickies in the long term, but could give an invaluable insight as to which you prefer to drive.
If your budget is big enough consider the 4WD versions of each. That would be a must for us.
Cheers,
Peter
OKA196 motorhome

you specify that the luton peak is a must an state it will be your forever home with little information, the luton peak is a bit like a roof top tent, good idea for young limber people but soon loses it's appeal if you have to climb up an down during the night.
some motor homes have electric beds that raise up over dining table that maybe an option you have not considered.

as mentioned plenty to hire as a test which is cheaper than a mistake