Do you think I should look at the AC Hi-Rail, DC Hi-Rail, or DC Scale Northerns?
Honestly, the best selection of easily available equipment is with HiRail. The common understanding is S consists of 85% Flyer/HiRail and 15% scale. This hasn't appeared to vary much in my 35 years in S and I'm in the 15%.
Most S Scale America (Des Plaines Hobbies) comes with HiRail wheels installed. Some don't include scale wheels. The former S Helper Service defaulted with HiRail wheels but had scale wheels in the box, except for the 2-8-0's. Undoubtedly, more of MTH's version of SHS were ordered with HiRail wheels.
And of course, AM can be ordered either way... However, right now the only Northern's in stock at AM are DC with scale wheels, so the secondary market is where one would have to look for a DC or AC HiRail version. The only difference between the AC and DC versions is the electronics in the tender. The AC versions have an electronic e-unit (and maybe a sound board of dubious effectiveness.) The DC versions have simply a pair of wires.