I was in the same dilemma. I opted for no bed over the cab for the following reasons...
*I wanted the extra storage over the cab.
*95% of the time it is just my girlfriend and me, I didn't want it to be too convenient to have to take her kids or my grandkids along. That may sound selfish, but I have the RV to RELAX, not babysit!
*Though my dealer said there isn't a noticeable difference, I would think the mileage would be slightly better as it's more aerodynamic.
*It looks to me the bed over design is a great way to catch bugs, windshields are hard enough to clean, I can't imagine crawling up there to scrub them off the overhang too.
*I like the sleeker looks from the outside and the skylight from the inside.
*I read a review online but couldn't find the link for you. It said something along the lines that with the bed over design next to the "table" sofa, that they kind of conflict each other and it isn't a great design. I'll keep searching and if I find it, I'll post the link. Good luck with your decision!
2014 Tiffin Allegro Red 38QBA