Backing out of a confirmed gig is not at all professional. A poor reputation is bound to be earned by doing this.
We all have had this come up and the higher paying jobs will come if you have a good rep.
Our quintet turned down a much better playing opportunity (more than double) but our word is important to us we will be doing the job we were already committed to.
If you can help the second offer line up someone good you will be on their "A" list for the next opportunity due to your good ethical behavior and your willingness to help them out.
Gainesville, Florida (USA)