To me, it's just a different way to get to Final Drive Ratio, which is all that really counts. If you are running 17.5 and aiming for a 4.0 FDR and have an Associated with 2 to 1 internal (ratio of the belt pulleys, by the way), then all you need is 2 to 1 at the motor pinion to spur gear. If you have the Tamiya with the 1.85 internal ratio, then you need to run 2.16 at the motor, which is also very easy. Now, when you get to Mod and you want to run like a 7 to 1, then having the higher internal ratio makes it a little easier to achieve. You don't have to run the huge spur gears. But in all honesty, the difference between 1.85 to 2.08 (Tamiya to Awesomatix) really is in the same ball park and they all work out without too much difficulty.

Now as far as the internal gear ratio, like I said, it's the ratio of the belt pulleys. You are limited to how large of driven pulley (at the axles) you can get into the car without creating issues like center of gravity or rotating inertia. On the smaller driving pulley, you can't get too small because of the limitation of the smallest radius the belt can achieve without creating inefficiency.

Heck, there are probably issues I don't even know about, it would be great if other people chimed in, I find it all very interesting.