Supplex is woven; Coolmax is knit
That’s the big difference, plus Supplex is nylon and Coolmax is polyester. Supplex is usually not stretchy, though it can be made with some stretchy material added in.
The knit of Coolmax will feel softer.
But the nonclingy nature of Supplex might feel cooler in hot weather. OTOH, Coolmax is a bit more breathable, which could help it feel cooler if you sweat heavily.
Supplex resists rips and abrasion better.
Supplex dries faster.
Supplex is wind-resistant. Coolmax is not.
If you are talking about paddling or hiking wear, I’d choose a baggy Supplex shirt WITH VENTS over Coolmax, for the 90/90 day you mentioned. Even if you soak the shirt with sweat or water, it dries more quickly than Coolmax does. If you get the vented long-sleeve shirt with roll-up tabs, you have 2 shirts in one. During the heat spell in AK last summer, I wore Supplex-type woven vented shirts and pants. In addition to drying quickly, they kept my arms from getting sunburned (no need for sunblock except on face) and kept the bugs from biting through.