Nylon/Cotton Ripstop is stronger, durable and will hold together better. On the other hand im not sure but it might be heavier.
Cotton might be more comfortable?
Not to sure about all of that but im pretty sure Nylon is stronger.
The nonrip-stop Twills are very durable. All my field uniforms were Twill and since they weren't inspection uniforms, they also could be tucked away if they had scuffs and other wear marks. Field X's were not too stringent as far as looks but you also couldn't use ratty torn up uniforms either. I had a boot camp set of Rip stop that had holes of various sizes that I had to repair long before I had to do any to the twills ( I traded 2 sets of rip stop for twills from a guy who was issued the twills and wanted Rip stop long before we had to have nametapes sewn on)
Twill is a thicker fabric and is used mostly for colder tempuratures(to my knowledge). Ripstop is a generally thinner material can be used all year round. I believe it has something to do with breathability, Atilla knows what I'm talking about.
Anyway, 100% cotton bdu's won't have as good color integrity, meaning the colors will fade faster than 50/50 NYCO or 65/35 Poly/Cotton.