Here in the PNW, it is not uncommon to find salmon or halibut as fish and chips. Both are excellent.
Haven't seen salmon for fish and chips yet here in Oregon, but most places have halibut, either as a more expensive upgrade or as the only option (such as at the brewpub here in town). Not sure if salmon would be good fried or not; I've always had it either baked, grilled, or smoked.
As for whitefish, personally I don't care if it's halibut, cod, Alaskan pollock, or even talapia, just so long as it's not overcooked. Can't really taste much of a difference. For fish and chips I usually go for the least expensive option.