From my experience, both from airsoft and from spending 8 months wearing nothing but BDUs (seriously!), I can say the 50/50 NyCo ripstop will be your best bet.
50/50 twill fades easier, and is very very hot in the summer.
I even tried to fade my 50/50 NyCo ripstop, and it was a no-go. That shit will hold its color when washed through everything, including rit color remover and a quite intense solution of chlorine bleach and water. Pure bleach will turn it a funny orange-ish color, though.
50/50 NyCo rip is what the Army gave me, and it breathes well in the hotter months. The Cotton provides comfort, and the nylon provides durability.
Polyester is not durable, and while a bit more comfortable than nylon, will fade and become easily damaged. It also disintegrates easier in MIL-H-23699 Turbine Engine Lubricating Oil and MIL-H-5606 Hydraulic fluid. The 50/50 NyCo rip holds up well against most fluids and solvents when compared to poly, but I don't suppose that concerns you.
Stick with 50/50 Nyco Ripstop and you will be ok! Very durable, breathable, and comfortable.
Buy a size bigger than you would wear. Wash it in a mild detergent in Warm water and rinse twice. Dry on HIGH HEAT in the dryer. Repeat, but use liquid fabric softener the second time rinse. This lubricates the fibers and will help them keep their shape and maintain the small ammount of elasticity they have. Translation = your sh*t will last longer*. After you do the initial wash, and wear it, just wash them when they need it. Make sure to wash both top and bottom together every time, so they don't fade at a different rate (this is a long-term thing).
*This isn't recommended by the manufacturer, but works best.
Oh yeah, don't use Febreeze on them. If they smell, wash them or deal with it.
