I won't belabor the points already raised...
Some other things to consider too are that for some reason, the 5th wheel business model is to install one or (usually) two AC units, but I rarely see 3 of them--even in huge 5ers. If you plan to spend time in the South in summer, this is a non-ignorable issue. I have a motorhome, so you know my choice, but I also camp with several families in 5ers and they are hot during the summer. I can't understand why the 5ers don't have 3 AC units, but it's a critical flaw in my opinion. So if you buy one of any size, make sure you can get 3 AC units. Related to this is insulation. As you shop around, try to understand the level of insulation in the different models that you think about (motorhome or 5er), and whether it will matter to you (hot or cold climates?).
Also, know that 5ers and pull-behind trailers are being built with lots and lots of goodies in them, so you can easily find them with very nice amenities just like more expensive motorhomes. The down side to that is that these things start to get heavy, so often I see manufactures struggling to maintain a reasonable balance between features and weight. This usually ends up meaning lighter floors, thinner internal walls, fake wood, less insulation, etc. This is not unique to trailers and 5ers, but I think they struggle with weight more. So be sure you have enough truck to safely tow whatever you end up with, but also appreciate the trade-offs. Having said that, ultimately this was a major factor that led me to trade-up from our first Class A gasser, that was admittedly an entry-level 35' RV at $80k new, to a diesel pusher Class A. I only owned that first RV for 6 months before trading up... took a small beating on that new-coach sticker price, but ultimately I'm glad that I did it because the diesel offers a stronger chassis, and thus has more quality construction.
The other thing mentioned above has to do with those who will travel with you. Motorhomes allow them to travel in style. Our kids lay in bed, watch TV, play on iPads, use the toilet, and have snacks while we are moving down the road. Our friend's kids are less comfortable in the back seat of their trucks. For long driving days, this is significant for us.. Of course there's the personal seatbelt debate in Class A's, and you can think about that, but ultimately most people with Class A's tend to allow at least limited flexibility in where kids and other family reside during travel. For us, this enables long driving days with no complaints, and that opens up a lot more travel opportunities for us.
I must say that I really do admire the flexible floor plans in 5ers though, and that their slide-outs tend to be much deeper because they aren't designed for occupancy when closed up. There is clearly a lot more living space in a 34 or 40' 5er than there is in a similar length Class A. (so why don't they use comparable AC?? Ugh)