I'm a former finish guy for a boutique acoustic maker who worked with nothing but nitro the entire time and I didn't have to think about how I'd answer this: poly.
It sounds the same on electric instruments as nitro just so long as you do it thin (not the way most makers do, loading it up until it's a couple of millimeters thick when dry) and it's far less reactive with stands, case plush, bag interiors. It's also a lot more forgiving of impact.
There's a time and a place for everything, and the time and place for nitro is on acoustic instruments.
Also, FWIW, "nitro finished" Fender basses are a thin coat of nitro on top of a poly finish. Even the old Fenders were a few coats of nitro on top of a thick coating of Plasti-Dip. There's never been a Fender made that started with a nitro finish directly applied to grain-filled wood, save possibly for a few Custom Shop pieces.