Disclaimer: Long winded here... my summary is the last paragraph if you want to skip the details:
I've been a TT owner and user all my life, except for the last year. We have our first 5er. I'm towing with the same truck as my last TT and now 5er.
Length:
Last TT was 35.5 feet long (tip to tip). This 5er is 41 feet long, tip to tip.
My overall length with the TT was 58.5 feet. My overall length with the 5er is 59.5 feet. (5er wins. 5 feet longer, overall length 1 foot longer)
Stability:
Stability, towing? Same truck is a 1 ton dualy. I used a 4-point Equal-i-zer weight distribution hitch system for the TT. My 5th wheel hitch is a no frills, simple hitch. Results, I experience no difference. However:
The TT receiver hitch had some slack in it where the pin went through. This slight slack in the pin hole would cause the hitch to move forward or backward when starting from a stopped position or moving to a stopped position. There was always a slight "bang" going on.
The 5er seems to make some clunking sounds every now and then too. Results (wash out, they are equal).
Wind? No difference. I have experienced the 5er being pushed hard by the wind, exactly the same for the TT. Never experienced sway with either one.
Breaks? The 5er has much stronger brakes. I can lock them up on demand and almost get whiplash. TT never locked up fully, but could feel the pull. However, my truck has the integrated brake controller which makes the trailer work seamless with the truck. It worked excellent with both trailers. In a real white knuckle panic stop, the TT responded perfect. (Wash out- both equal).
Hitching?
After years and years of TT hitching, the Equal-i-zer system was by far the easiest. Switching to a 5er was a hard-knocks experience. My first obstetrical was figuring how how to reach inside the bed of the truck to reach the coupler handle, and the break away cable. With dual wheels and a bed that sits high and being short, it's impossible to reach inside from the ground. A 4 foot step ladder solved that. But it took time to figure it out.
In my case, hitching was easier with the TT. Although I've adjusted to the 5er, I think the TT was easier. (TT wins hitching up).
Backing up:
I had a bad learning curve with the 5er. TT was easier. But, I'm getting better after a year with the 5er. With a longer tow vehicle and a longer wheel based 5er trailer, the backing response is definitely slower at first, and then the response is faster. It just takes time to learn the difference. But, I've eventually been able to back into every campsite we've been to. (Backing -- wash - equal, just different).
One thing I wasn't prepared for with the 5er was the distance between the truck bed rails and the bottom of the 5er overhang. I have about 6-7 inches clearance, but I also never realized how the side-to-side movement of the camper to the truck can be. The underside of the 5er had contact with the truck bed rail (once) and cracked the fiberglass. I was able to repair it myself, as it was a surface crack, but still, it made me keenly aware, I never had this problem with a TT.
In this case ... TT wins by a land slide. The TT can tilt side to side all it want's and never make contact with the truck.
Space?
I always said I'd never give up the construction workers shell on the back of my pick-up truck. It had panel doors on both sides and 2 full length (8 foot long) shelves, for a total of 32 feet of shelf... off the floor bed of the truck. With the shell and those shelves, it afforded a LOT of storage for a LOT of stuff, keeping everything dry. In addition, I had ladder racks on the top for transporting a canoe.
Well, with the 5er, the shell was given away to someone else. I did get a cover for the truck bed to hide the 5er hitch and keep it out of the elements. I have since found alternative locations for everything I use to carry in the truck, which was all for camping anyway. We quit carrying the canoe about 4 years ago. We're getting to old to man-handle it any more. So, really, no need for those ladder racks. So, after finding new places in the 5er to store everything, getting the cover for the truck...
(Storage Space --- wash out -- same, equal).
Now I could ramble on for another 15 pages about all the interior differences, but really, each floor plan of every camper, of every model has it's own unique attributes. You either love it or you don't. No comparisons there.
Thanks for enduring my "thesis" ... but to summarize ... both are equal if the tow vehicle is a one-ton dualy and the TT is using a weight distribution system with built in sway control. If you are using a lesser tow vehicle, you may experience some very noticeable differences.