Marines will probably find out where they're going near the end of Time Between Saturdays. You'll most likely be asked for your preferences, but "volunteers" will be asked for to fill the appropriate slots for TW-4,TW-5 (Corpus and Whiting respectively) and Vance AFB. You'll be told some BS about if you go to one or the other you'll get to start sooner, later, you'll definately get jets, C-130's, etc... All lies.
There is no advantage to starting earlier, going to one or the other. EXCEPT: If you go to Vance you'll have to unlearn certain things about how the Air Force does things that the Navy / Marine Corps doesn't. Air Force: rigid syllabus, stand up EPs, there all day, but at least you know what's next, mostly. Navy: flexible schedule, no stand-ups but you do your EPs in the plane, the disadvantage is you're totally on your own to learn your stuff.
And one other thing, when you get to NASP and check -in to MATSG-21 you'll probably have your API start date changed at least once, and where you're going for primary change to. Get used to it.
So My Advice: Don't go to Vance, and spend as much time in the pool before API as you can, you'll be busy enough later. And stay away from the teenies at Seville on Thursday nights.