If your primary interest is domestic upgrade, I would recommend you choose Continential/Northwest Airlines. (CO and NW are alliance)

Chicago is AA's hub. There are too many top tier elites who are willing to pay full fare. Since you are a student, you are likely to purchase discount coach tickets. Your chance to get confirm upgrade on AA are not high on popular routes like ORD- New York.

CO's upgrade polciy is based on availability, and it won't cost you anything. (AA needs either miles or Electronic Upgrade). You have better chances to sit on CO first than AA.

If you plan to fly internaional a lot, then it's a different story.