Perhaps I'm the only one agreeing with OP with his self assessment that he's probably over-analyzing things. I also agree with the previous poster that picking a FF program is not a lifetime decision as one should examine and adapt to his situation change and new learning along the way.

OP, you stated your desire is to get seat upgrade. You said you could reach the lower elite level on the legacy carrier(s). Its not clear to me whether you have assessed the likelihood of getting a seat upgrade for lowest tier elite. Another thing is that your routes (at least those you described) are short, so while getting upgrade is nice but is it that a meaningful benefit to you (considering you have to "work" for it)? You are in your first year of college, your flying pattern would likely change after you graduate, etc., you have to assess the cost/benefit for your individual case.

p.s. We should recognize people have different needs and wants - I know of a few who love DL because of their frequent service (almost hourly I think) between Boston and New York, and it's perfect for them because of constantly changing work/meeting schedule. I also know many people who hate DL because of the poor mileage program especially with regard to redeeming miles.