I think the sweet spot is 18' 10k. That's what I bought, and I love it. It doesn't sway, it pulls straight and steady, and the brakes are strong.
The 10K was only a few hundred dollars more than the 7k, and it only weighs a few hundred pounds more.
A built XJ packed full with wheeling gear will be about the max a 7k trailer is rated for. That's fine, and it won't really be a problem, but I like the extra security of the 10k.
Also pay attention to things like the tongue jack, ramps, lights, wiring, tires etc ... when buying a used trailer, those are the things that can nickle and dime you if they aren't in good shape.
Lastly, I prefer a wood deck over a metal deck. I put my Jeep on the trailer to work on it, and I just prefer laying on a wood deck more than a metal one.