I know it sounds silly, but a few factors come into play here.

If you're trying to maintain a certain speed, regardless of gearing you are going to have to spin faster to maintain it, even in a different gear. Lighter would be better in this case.

I'd mostly be concerned about bike fit, tire size, etc. If he wants to go slower he can just pedal slower. His "mashing" and "spinning" are likely not all that different.