Since you are working out the towing math, my two bits of advice on which trailer is....
1. Make sure the bathroom is comfortable - you can sit on the toilet comfortably, the shower stall is roomy enough, and the sink is usable. We love our TT, but the toilet is too close to the wall, the shower is tiny, and the sink is hardly big enough to wash your hands in.
2. Get what the wife likes. Old, old wisdom dating back to the time of cavemen - if the wife ain't happy, no one is happy.
As to future planning - once kids hit 16 years old, it is common that they start to be less interested in camping with the parents. We bought our 21 foot TT 14 years ago, and now that the kids no longer (usually) camp with us it is a beautiful size for two people. Except, of course, for some issues with bathroom size! lol
Have fun, maybe see you on the road!