You MUST take the Spirit class date. In this industry you never, ever know when the music will stop. If you turn down Spirit thinking you are going to take a Sept JB class you are one stock market crash, oil bubble or one terrorist flying an airplane into a building from being stuck at your regional.
Take the Spirit class, reevaluate two weeks before your JB class date.
Spirit will probably have faster seniority progression anyways, and a new contract.