Punki - if your concern really hinges on the "cap" for credit card miles, I'd say stick to United. As I recall there is no cap on the United Mileage Plus First Card miles if you're Premier Executive.

(I just pulled up an application, and the fine print says, inter alia, "The maximum number of miles you can earn with the Mileage Plus First Card is 10,000 miles in a single billing period with a maximum of 50,000 miles per year. For Mileage Plus Premier or Premier Executive there is no maximum number of miles you can earn per billing period or per year.") (Emphasis added.)

Another thought - if you don't already have one, you might want to consider geting a Diners Club card. Not only do you get 2 points per dollar (which convert to at least one UAL mile per dollar), the points can be used on other airlines as well, and you have a two billing period interest free grace period you can use for "float". If you are interested, please let me know and I'll sponsor you (you get 24,000 bonus points the first year at the rate of 2,000 points for each month you charge something on the card, and I get 6,000 pouints for the referral).