There is no more room In a fiver than a DP MH. If both are the same dimensionally then they are they same. There is no magic wand making the fiver bigger. This is a urban myth. Of course disregarding the high ceilings in the fiver ( which is not usable other than a ceiling fan).
That being said, we've owned 4 fivers & now our MH. We have plenty of livable room in our MH ( much more than our 40' fiver had - we are 41'8" with the MH). Think about opposing slides in a MH - they really open it up. Some even have full wall slides ( as do fivers). It boils down to how much you want to spend and how you wish to travel.
I thought I'd never own a MH - but after having this one we'll never go back to a fiver or travel trailer. Much easier to get ready to go. More fun driving, the pets enjoy the space when traveling, not as exhausted after arriving, wife is much more relaxed, just some things that matter to us. Plus now the truck doesn't just sit in the driveway ( as it ended up doing with the fiver).