It depends on what you want and/or need. The SM420 is a very sturdy box that was in production for about 30 years and found service in everything from light trucks to busses and military vehicles. It was, and remains, an excellent piece of equipment. But for all practical purposes it's "only" a fully synchro 3-speed with an extra low first gear if you need it.
The T-5, on the other hand, has 5 fully synchro gears including an overdrive if you have a need for expressway driving.
So, if you find yourself wishing you could downshift into first or upshift into overdrive the T-5 seems to be the weapon of choice. Otherwise the SM420 is going to give you excellent service and outlive you and your kids.
I want my truck to be a 1951 Chevy truck that's fun to drive on local roads and around town so I had my SM420 overhauled and will keep it.
