It's just different for everyone. Where do have to park it when not using it? Where do you want to take to use it?
Also, watch out when people advertise lengths. Some will list the brochure length which usually excludes the tongue and bumper while others will list the true length. And most importantly, don't trust what anyone advertises. Take a tape measure with you.
I was in a 31' (bumper to ball) TT and needed something bigger. Looking at the 35+ footers I realized they are just too long for where I park and how I use it. I went with a 36' 5th wheel that is only about 1-2 feet longer behind my truck than my TT. There are tons of pros/cons between 5th & TT. I absolutely labored over the decision for months before finally going for the 5th.