You will likely get different opinions from different people here based on their own experience.
From my experience I recommend investing in a course that covers BOTH systems right from the start. Learning one first then moving on to another will usually mean it takes longer to master both systems. There is so much information that pertains to both systems that learning both together on a course that is designed to teach you the benefits of both, as well as which system to choose for each individual client will be better value for money in the long run.
Are you based in the UK?
In the UK CND offer 2 courses that would teach you both systems right from the start. CND Enhance will include both systems and CND Complete will include both systems but ALSO CND's natural nail services; Spa Manicure, Spa Pedicure, CND Shellac and CND Vinylux.