I had a 28' trailer with a 1/2 wide bunk on top over a double bunk on the bottom. My kids hated it mainly because they didn't like the opening along the side of the top bunk. I put up a rail, but that made it more difficult to get in /out. It was also apparent that the upper bunk would be small as they got older. It was tight enough that I wouldn't want to sleep in it.
When we shopped for a new trailer, one requirement was double over double and the kids love it. They both have enough room to spread out their stuff so the bunks are now almost like individual bedrooms from their perspective.
One other note: check the headroom on the upper bunk...a lot of them are very tight. The lower bunk gets lots of headroom but the upper bunk is usually quite a bit tighter....that was another problem with our previous trailer (...and it seems is always a problem with the triple bunk layouts).
