I have experience with the Mitchell and the T-5.
I have a Mitchell in my '29 Phaeton and have driven it over 95,000 miles. No issues and very durable. I like the splitter and have become proficient in shifting without grinding gears. I would do it again.
I installed a T-5 in a recent build and the transmission works good but it caused issues with the brakes. I tried to set the car up with Ted's latest floater kit and failed. The reason, was that the T-5 kit replacement service brake cross shaft is moved back a few inches from the stock location. The original location of the cross shaft is at the u-joint coupling centerline of movement. Having the cross shaft moved back introduces some movement in the brake rods when the rear of the car raises or lowers when going over road bumps. Ted's Floater kits rely heavily on proper brake rod length adjustment when setting up and operation. This varying geometry in the brake rods, when going over a good sized bump, would apply the brakes when the car body was on rebound. I tried multiple times to adjust and had some Club members with much more experience then I had with floater kits help and we all failed. I switched over to hydraulic brakes to solve the problem.
IMO: The T-5 is still the best option that I am familiar with for the car I installed it in based on how I plan to use it. I have a "B" engine in this car that produces about 80 hp and wanted the extra strength of the T-5 transmission and wanted a synchronized gear system that is friendly to an inexperienced driver (wife).
The time spent installing a T-5 kit is 4 times the work of installing a Mitchell and for me, after installing the hydraulic brakes, about $2,000.00 more in cost.
Good Day!