Ask what the OITC of each window is. The higher the number the better sound dampening the window has.

I suppose my real question comes down to this: All things being equal, whether a 1/4" of additional laminated glass thickness is better than a 1/4" loss of dead airspace between the two windows for purposes of sound abatement.

You are way to hung up on glass thickness and dead air space, when what you should be focusing on is the laminate thickness. A laminate thickness of .90 mil is going to give you greater sound abatement than a .30 mil. As I said before, you need to ask what the OITC is. There are other factors that determine STC other than glass thickness or the size of the gap between 2 panes of glass. Varying the thickness of the glass is also a factor in sound abatement. Again, you need to find out the OITC and quit driving youraelf crazy trying to understand something that has little consequence. If the salesman continues to dodge your questions then tell him to give you a call once he finds out what it is and in the meantime, find another company that has a salesman who knows what he is selling and can answer your questions. This guy you have now sounds more interested in selling you something than in solving a problem. Also make sure you get the OITC and not the STC as the STC in your situation is not as important.

I hadn't considered the laminate thickness. That's a very good point. When you say varying the thickness of the glass, are you referring to varying the thickness of the two panes of glass in the laminate?

STC/OITC is affected by 3 things.

Mass, Airspace, and Thickness.

An object with more mass will be better at blocking sound.

The larger airspace will be better at dissipating sound.

The thickness and varying thickness of the lites (glass) will cancel out a larger frequency range.

Here is a link that might be useful: STC explanation