If you plan to use the Coticule for full razor sharpening, I advice either n°58 or n°63. Both are excellent Coticules. n°63 could prossibly be just a tad more lenient for getting easy results, but in experienced hands, n°58 would probably leave a mellower edge. Which can be an advantage for people with very sensitive skin, because nothing is easier on the skin than what I call a "mellow" edge. But the difference will be very small and the properties of the razor are at least equally important in that department.
Kind regards,
Bart.