Flat gears are what they sound like they're "flat" they go up and down like you would expect a regular gear to be (ie. like the gears on a bicycle).
Helical gears on the other hand are "helical" or "angled" like in this picture:
They will make your mechbox quieter and "mesh" better if you shim it right.
As for the "torque ratios" normal is what you expect, they're normal (I don't have the ratios off hand but basically they provide "normal" amounts of torque). So I guess you can think of it as, high speed gears (high speed for high ROF but will not have enough torque to pull back a stronger spring), "normal" gears that provide normal ROF and normal amounts of torque, and high torque gears that provide high torque but low speed. (Note I use "normal", "high", and "low" in a very loose context to illustrate the idea, IIRC Systema or something makes this "high speed and high torque" gearset (possibly motor) I was looking at it last year but I forget what the deal was with it.