By saying your spa is part of the pool, usually indicates you have a spillover type spa.  The spa can only be heated to higher temps if isolated from the pool water and circulated through whatever heater you have.

Solar just for a spa will nearly be the size and cost of one capable of heating both the pool and spa.  So, if you are going to do it, you might as well consider one large enough for both.

The spa heating by solar can be automated and prioritized by using automated controls, but would still only be able to heat the spa during the day and kept covered to prevent heat loss for night time use. Some of this could also be affected by you already having some type of pool/sap automation to control changing from pool to spa mode at preset times. 

If absolute guaranteed use is required at all time, the gas may need to be used occasionally or may a heat pump might be a better option depending upon the cost of electricity in your area compared to gas.

As you can see, there is no simple answer to this question.  It depends upon how, and when, you use the spa and what your expectations are.

For more information, please visit The Energy SuperMarket