Although I have never driven a TR3 with a 5 speed, I have driven a TR6 and an MG Midget with a 5 speed conversion, as well as lots of British sports cars with stock four speeds, and 4 speeds with o/d, as well as my old Austin Healey Hundred, that had the early 3 speed with overdrive.
I love the Laycock overdrive, it adds immensely to the driving experience (both in kicking it in and out of O/D and listening to the exhaust note change, and the more relaxed cruising) and to me is one of the things that make old British sports cars unique and cool.
A five speed is probably going to be more reliable and efficient, but I don't think reliability and efficiency is what driving an old British Sports car is all about.
Everybody has their level of mods and upgrades they want to go to, from as it left the factory, to its OK as long is it looks like it left the factory to Pertronix and internal engine mods or ok, or period mods, to fitting a five speed, or fuel injection, to fitting a Honda 4 or BMW 6, or Rover/Buick v-8 or small block ford.
To each his own, and I don't mean that sarcastically or condescendingly, but I do like the O/D, try it you'll like it.