I have 18' (16' + 2' dovetail) 7k trailer brakes on both axles. It functions ok, but if I were to do it over it would be 20' (18' + 2' dovetail) 10k trailer. Since op asked 16' or 18' as choice....18' with 10k axles would be minimum with 20' length better.
The weight of trailer between the sizes is minimal. At times a good trailer has features that can make use a better experience rather than a pain in the rear. I have three different types of trailers: hauler, dump and small aluminum utility....they all work well, but with experience would alter each one if buying again...experience as teacher.
A couple things I did learn from first cheap landscape trailer I had to current hauler was: slide in rear ramps with the ramps being reinforced smooth plate on top and not ladder style (allows loading small tire items) 16" wide vs 12" , and tapered bed on front along with tapered tongue tool box.