I actually have both
My "new" mechanical pump has never had gasoline run through it but it has been installed on the engine for the last ten years.
I also have a spare electric pump in my tool bag.
My brother initially used the old electric pump as a booster installed inline with the mechanical pump and mostly used it on long inclines and whenever the 40 vapor locked, but eventually kept it on all the time.
When I got the FJ40 running again I followed his habit of running the electric all the time, but I had bought a spare electric pump just in case. This worked fine for two years, but then the diaphragm on the mechanical pump failed and the electric fuel pump happily pumped gas directly into the oil pan and the forty stopped dead on a hill.
I had enough spare fuel hose to bypass the mechanical pump, but the electric pump didn't have enough umpf to keep the carburetor bowl full, and it was a long drive home having to start up again every few miles after the truck stalled out.
I bought a new electric pump and new mechanical pump, but this time I hosed the electric pump directly to the carburetor and left enough spare fuel hose on the mechanical to tie it in if the electric ever quit. I then also bought a spare electric pump.
No fuel pump = no driving!