Your 2 main choices here are UA and AA - look at the stickies at the top of each forum to get an idea of what benefits each program has and then go for the one best suited to your needs.

I strongly suggest you pick 1 and stick to it AND that you consider throwing in a few w/e mileage runs to get you to top tier (1K on UA/Exp on AA) as this commute will easily bring you to mid-tier, but only tantalizingly close to top tier unless you throw in some addtional flying.

Be warned that you are picking a particularly problem prone city pair - snow in winter (eg today) and both cities prone to thunder/lightening in the summer.

Happy flying!