Selected by Navy and AF JAG. Direct Appointment

Navy has already sent paperwork to MEPS no date yet. I prefer to go to AF JAG, but, I haven't heard anything from AF since the FSO called to congratulate me. I want to be a JAG, so I'm concerned that if I stop process with Navy and AF falls through, I will lose out. Also, what happens if I go to MEPS under Navy application and then opt for AF, will that mess me up? I have no military experience, I am just a lost civilian who wants to be a JAG! I am open to all advice regarding choosing a service, and/or how to tell Navy I'm out, without, burning bridges!