is it true that the higher the voltage the less punch?
I am no electrical genius but I think you want to keep the voltage rating as close to what enviorment you are operating in. For example, a 10V cap on a system that runs 7.2 volt, a 16v cap for a 12v system etc.
The uf rating is the capacity of the cap. The larger the capacity, the longer it takes to charge the cap (which I think is true for the voltage aspect as well) between discharges. All the manufacturers like NOVAK claim that the 4700uf cap is perfect for our RC stuff, I have read where some guys run 10,000uf and in an extreme case of experimenting I have seen photos of a guy with a 25,000uf cap that was like 4 inches long!
I am going to try the 10,000uf cap when I test a 6 turn motor I bought just to see if it will work. I will also test the 10,000 cap on my standard 19T as well.