Don't join oi do something just to look good. Folks see through that. Do something you have a passion about. If you already participate in something you're passionate about where you can show leadership skills, do that. To answer you're direct question I don't think USNA would value one of these specific activities over another.
While Sea Cadets are a great organization they only give you leadership experience you can get other ways. As a junior you wouldn't be in long enough to rank up to a place like Chief. Promotions in Sea Cadets can only happen every 6 months and involve coursework, testing, and advanced training. You would really have to push. My dd didn't join League Cadets because she wanted to go to USNA, she decided that after through the experiences she had had with our squadron.