I would recommend Continental Airlines. I like their OP FF program as there are many ways to earn miles if you are on East Coast. Also, their fleet is new and I have heard good reviews about it from my family members who traveled.

SPG transfer rate is same 2:1 for CO and UA. So that will be same. Also, the flight time is less with Continental. Their price is better compared to United in March (I just checked a sample data 14/mar to 28/mar).