The higher the ratio the better. Kind of straight forward right?
Think about it.
The more sensitive the tuner, the more easily you can get in tune. Correct?
Then of course, you will have to take more turns on the tuner pegs... which can be tiring.
But its okay. Being in tune is > than tired fingers no?
I've used 21:1 ratio tuners before, very nice, very easy to get in tune... but I need a peg turner or I'll be too tired. Also, if you are using a floyd rose system, you don't need the high ratios.