Well, I assume someone here is more knowledgable on the officer side than I am, but I will offer that if you are dependent-free and basically free of debt, and young, then the Academy might be a good deal. Of course, you basically get to play boot camp all over again and all that jazz.
I would look more closely into STA-21. If selected, you get paid at E5 (or your current rank) for 36 months while you complete your degree at the university of your choice. You will go to school tuition free, but must pay for books.
My inclination is to take STA-21 over USNA. Prestige is fun while you're in, but I figure once you are out, the difference is nil. In fact, I am sure that there are schools looked upon more highly than the USNA in engineering fields.
As far as I can tell, the degree is the defining line, not whether you were an officer or enlisted. If you take advantage of all of the opportunities open to you, there is as much benefit in staying enlisted as there is being a nuclear officer. Being a nuke officer is NO walk in the park.