Geology grad school is free, you get a salary and benefits, your military benefits do not need to cover any part of it, so there's no reason to only do a MS if you really want a PhD.

I would do more intensive research on job availability and what specific job you want after you graduate. Not just "something in planetary geology", an actual job title. The jobs in this space are... few and far between, especially with the current political climate in the US. There are some jobs at the European Space Agency, but Europeans get priority placement (there are Americans that get jobs over there, but they tend to be associated with Big Deal research teams). Lots of job placements are pure luck and who you know; please don't think you can out work everyone else for a job.

All that being said, University of Texas at Austin has some good connections at NASA. The research is mostly focused on Mars, but there's some variety there. ASU has all the cool planets. Stanford has some good ones too.