If you Google those questions, you'll be able to find quite a bit of information posted online and in various forums. To get a scholarship or advanced standing as a college programmer, is quite competitive. My son was recently told they are handing out 40 marine option scholarships and 15 advanced standing nationwide. Or maybe it was 30. Lol
He was also told this is what he will be competing against for MO. I'm sure if you look you can find similar information for the Navy.
Average SAT: 1313
Average PFT: 287
Average GPA: 3.45
Average ACT: 24
The Navy puts a lot of emphasis on the degree you are going for. Not so much with the Marines. Even beyond academics, there is a difference in how they train and in their mindset. You need to research all aspects of this to decide which one may be a better fit for you. Chances are if you are not happy with your decision you won't do well enough to earn a scholarship. My guess is how competitive it is to even join may vary from college to college. At my sons school, there is only one sophomore marine option college programmer. And very few freshmen. I do not know if it has anything to do with how selective they are with admitting them, or if it's just that's how many wanted in.
Good luck with your decision.