I have a choice of routes, but the one I use most often is very similar to yours--16.5 miles, about 400 feet of climbing, half road, half paved MUP, and the MUP can be rough with tree roots corregating the surface. (But the roads are nice.)

First, I've found Continental Grand Prix 4-Seasons to be just as trouble-free as Gatorskins, but they provide a considerably more comfortable ride and they're super grippy in the wet. The downside is that their thread life is somewhat less. I always found the Gatorskins to be too skittish in the wet, so I'm willing to give up some tread life for better wet grip and better ride all the time. YMMV.

I've run GP 4-Seasons in 23, 25 and 28. There is considerable improvement in ride quality as you go bigger, if you adjust air pressure accordingly. However, I've not found any difference in either speed or climbing ability between them. Since there's only a 30 gram difference between the 23 and 28, there's no real penalty there either.

So I run the biggest ones that will fit in my frames--28 under full fenders on the commuters, 25 on open wheels on the roadie.