Which lasts longer, wider or narrower seats. My old machine shop says narrower. My new shop says wider. Who is right? Is there a different question I should ask?

Wider for sure. Narrower flows better which is why race cars use them and they arent putting a million miles on the seats. Some even use practically a perfect radious! Which is reeeall narrow.

It pretty much depends on what they are calling "wide or narrow" :D
We have no problems cutting the intake seats at 1.3-1.5mm (.051-.059") & the exhaust at 1.6-1.9mm(.063-.075) for almost everything except some of the smaller foriegn stuff which can be quite narrow. Like under 1mm (.039")
Real wide seats lower effective seat pressure in PSI which can cause crap to get stuck & keep the valve from seating properly.
We do use slightly narrower seats like 1mm for some performance seats in some applications also.

use narrow on the street so they last longer..........soon or later they will be wider after all the high rpms......go wide now just more short lived life ............lol .....logical right??????????

but then again i cut my head at 45 and my valves at 44 and high on the valve cause i know i will beat them in the head.........who needs harded seats.................lol

you want me to do you machine work? lol