The 85mm F1.8 is a decent lens at a good price. Said to be very sharp.
The 85mm F1.2 is a professional lens with an ultra wide max. aperture and costs an arm and three legs.
The 100mm F2.8 macro is a sharp lens with the ability to focus close enough to achieve 1:1 macro magnification.
85mm f1.8 - Excellent portrait lens. 1/2 body shots. Good indoor sports lens. Fast with quick focus. Tack sharp.
100mm f2.8 macro - Main purpose= 1:1 macro. Secondary = Head and/or head and shoulder portrait lens. Tack sharp. So-so sports lens. Slow focus with a very touchy focus ring in manual focus. (it's designed for macro) Not as fast optically but still nice.
My portrait kit includes a 50mm f1.4, 85mm f1.8, 100mm f2.8 macro & a 135 f2.0L.
My macro kit includes a 100mm f2.8 (same lens) set of kenko tubes for closer than 1:1 and a novoflex rail system.
It's not the scant 15mm that matters it the purpose and design of the glass.