Re: Impedence Switch
Since you have the option of 4, 8 and 16, I think you should ask him to do all three. In my Edana, you have one hole in the back that the old style selector goes. Next to that are two speaker outputs. Total of three holes. It sounds like your amp guy is going to use the same number or less (2 holes?). I'd lobby hard to do all three. Mojo sells a heavy duty old style impedance selector switch that is very nice. I think they're less than $10 bucks.
Other than that, I'd take 4/8 over 8/16, but that's just me. There's a crowd out there that say to always use the maximum number of windings, maximum impedance if and whenever possible 16 in your case. I don't think it matters as much as a lot of other way more important factors inside the amp.