How does the F+T transmission compare to a Sherman? I have a parts 9N that has an F+T, and have wanted to find a Sherman for my 8n.

Ben, where are you located? I'm in SE Michigan about 40 miles north of Toledo near US 23. I have 2, 3 speed Shermans that I want to sell as a pair.
One is in the original crate that I received it in from John Smith several (many) years ago. The other is on the shelf and the seller told me that there is a piece missing inside.
I also have a non-running 9N with an F&T transmission and another with a Hupp 3 speed.

An F&T overdrive and Sherman overdrive do the same thing: add three forward and one reverse gear by turning the transmission at a higher speed. The difference is that the F&T is more like the old tractor transmissions where you slide gears back and forth to switch speeds, requiring you to come to a complete stop. A Sherman is more like a modern transmission with fixed gears and you move synchronizer collars to connect gears to the input/output shafts. With a Sherman it is better to shift after stopping completely, but I have shifted on the move as well (though I would not recommend it especially since some parts are no longer available if they are damaged). As far as durability, I would say both are pretty durable.