I signed up for both Delta SkyMiles and United MileagePlus since I traveled on both of them within the past 30 days. Apparently I already had a United account but I had never used it. (I don't think i've traveled on United in about 4 years).

I checked out the other thread and it was useful reading.

I looked at a few credit card offers which I could get for either Delta or United, but I would have to pick one for it to me worth my while to get enough points. Probably leaning towards United which has better international destinations within the Star Alliance that I would actually travel to.