[asid61]

As far as custom gearboxes go, I like ball shifters better due to their decreased footprint (as there is no dog in between the gears) and their slightly decreased weight. See 192's 2014 gearbox for inspiration.
For COTS options, I believe the WCP 2-cim shifter is a good option due to small size and weight. Their 3-cim shifter is also quite good. I somewhat dislike the Vex options, because the 3rd stage makes the gearbox much larger and it takes up a lot of space to begin with. It's heavier as well.
On the other hand, the Vex version is enclosed, making it lower maintenance. And the space/weight cost isn't all that great.

I didn't see this answered above so:
Spread ratio is essentially the ratio between the high speed and low speed. So a gearbox made for 18fps and 6fps would have a spread of 18/6 = 3. A spread ratio of 3.