As many have mentioned, the difference between the two types of engines as far as longevity is disappearing with the new regs and tech development. Someone else also mentioned a situation nearly identical to ours - IF and I mean IF diesel is available I've seen the prices fluctuate wildly. I've seen it as high as $1.65/gal more expensive than gas. So the diesel vs gas question is important to us.
Floorplans don't vary by much in Bs. There's only so many permutations of layout in that limited space. We like the twin beds because they can do triple duty as passenger seats and banquet seating with a table in between. Most of the time we're outside eating so the twins can be beds or passengers.