You might want to think about full on gas since you have it in the home already for your area as below a certain temperature the electric heat strips will be kicking in so you'd be using a lot of electricity.
Also, you may want to think about only adding one system for the additon and put two or three thermostats on it, effectively "zoning" the system. You'd need to obviously have a zone control board so study and research "air conditioning zoning." If efficency is what you're looking for than this would be the way to go. This link should get you started.