[Manowar]

Skip,
You seem to want to just boil it down, so here goes. The single plane design, (Jr.) will make a generally higher figure at generally higher RPM.
The dual plane (Air Gap) will be generally superior lower-down up to an RPM point when the Jr. will make a bigger number and go higher up in the RPM range. (Assuming your engine can.) If you were told this about your spec engine, you haven't told us what the specs are, so we can only generalize here.
If you have say a 9.5-1, 351 that goes to 5200 with a dual plane, you'd waste money going to a single plane because it generally won't make a superior HP number until after your valve train quits at 5200. If you had a 7500 RPM valvetrain with a dual plane, you'd be choking the engine 2000 RPM before the valvetrain quit.
The dual plane will feel much better (torque) in the 5200 RPM operating range described, with a probably higher HP number than the Jr. The Jr. allows you to install more cam, heads and carb to fully take advantage of the breathing. The gap under the runners means nothing in this comparison as both types have one.
To make a wise choice and not waste money, you can't simplify so much-you have to look at the whole combination and RPM envelope you want make the most power in.
It's a compromise.

Manowar