Per foot of total length you do get more living area in a 5th wheel than a MH.
There are advantages and disadvantages to both. But newer 5ers are now available with automatic leveling systems and power awnings so the set up time is not much different than a MH. Unhook the 5er from the truck (if you want) and push a button to level. On a MH you disconnect the toad. From there it is pretty much the same, hook up water, electric, sewer.
MH has the advantage that you can get up and walk around while traveling. 5er you have to stop and get out if you want to get to the RV.
One factor is your travel plans. If you plan on a lot of miles and road travel days, a MH may be a good choice. If you plan on staying in one place for long periods and only occasionally moving to a different area, a 5er may be the better choice. In our case, we only travel 2,500-3,000 miles per year and only about 10-12 road travel days. Did not make sense to have a MH to be on the road 10 days and sit idle for 355 days per year, and so few miles per year.
An advantage of a 5er. Our truck died on the side of an interstate a few miles from our destination several years ago. Had the 5er towed to an RV park and the truck towed to a dealer for repairs. Got a courtesy car (under warranty) and vacation had minimal interuption. Had it been a MH we would have been without the RV for a few days.