New prospective TT owner - how to pick a high quality model?

If your truck is rated to handle it, fuel economy of a heavier trailer is not that big a deal. It is the frontal area exposed to the winds which slows you down, not the TT's mass.

When in doubt, go bigger, not smaller. With a family of three, 24 feet can get cramped after a while.

There are a lot of floorplans out there. I'd probably go shopping to narrow down the models down that are usable, and go from there.