i basically fix all computers, for 6 years now, and if it's about making money, fix the pc side since they have more problems and more computers
but if you want to have the basic knowledge on macs, get certified or work for an apple certified reseller, apple store, or apple warranty company
many people find that the hobby of macs is very different than the day to day realities of pleasing paying (mac) customers who may sometimes become impatient and irrational and require you to work on their machines strange hours of the day or night
and realize that not all mac users know how to keep their machines relatively trouble free and may really gunk up the machine before you get to look at it