What I do is follow the spelling that is presented in the original Christening record. If it is McCABE that is what I enter.
I must say though that in all my Mc names they have all been spelt with the Mc and a capital letter to continue the name whether you spell it with all Upper case (as I do ) or not.
So it would be McCABE or McCabe either is right, I just like to make all Surnames stand up to be seen easier.