Unless your VISA united card is different than most every other card, your "points" are 1% (or maybe 2%) of your purchases with the card. The difference between an airlines-sponsored card and another "cash-back" credit card is you can only use your points for airline tickets. So, when Capital One advertises "no black out dates on our miles", what they mean is they'll give you 1 - 2% back as cash which you can do whatever you want with. Buy an airline ticket or buy lunch. They just call it "miles" to get the attention of folks who have airline credit cards. "Double miles" = 2% back on some purchases instead of 1%. So, which is better? A percentage back you can only use in an airline's frequent flyer program, or the same amount as cash?
I do have an American Airlines / Citibank card, but mainly to get the free checked bag (for my scuba gear). Accumulating double "miles" when I buy AA flights is fine (think 2% of the purchase price), but I use other cards for most things. The nicest I've found is the Amex Blue which gives 6% back on groceries, 3% back on gas, and 1% back on everything else. It adds up pretty fast in our household. The annual fee is covered many times over by the cash back credited directly to the billing statement.
"YMMV"
