A 40' x 8' box is the same whether it's a 5er or a DP. They can both have the same extra footage in slides. Where a 5er can seem larger may be in sleeping accommodations in the toy haulers. You can sleep ten people in them, then when you take the toys out, you have a large rear play area for the kids if it's raining out, and the fold down tail can make a nice deck. It all comes down to which floor plan works better for you.
We have a 39' DP with three slides and chose it over a 5er. We're happy and will keep it. I do believe the MH is faster to set up and break down, especially in bad weather. Our Jeep is faster to un-hook or re-hookup than a truck.
Bill