I have to agree with kincanucks, here. Go and get your high school diploma. You are probably only short a few credits and yes, a diploma is
much better than a GED. For example, if you did want to apply for university/college course, they may require specific prerequisites which a GED does not have.
As far as I know, the army will not "get you your diploma" while you serve. The BPSO is a great source for courses, however, in your first year or so (depending on trade choice) you will be too busy on BMQ and trades training. Also, there are opportunities to go and take courses on your own, via distance education or by going through an adult high school where you do assignments on your own, hand them in to be marked and only have to be present for the final exam. The latter one is what I did when I went and got my Grade 11 Advanced Biology (at the ripe "old" age of 38

).
In my opinion, the recruiting centre will look more favorably on you with a diploma vs. a GED.