I have found that cards such as Chase Sapphire and Amex to be very valuable in that the points you earn are transferable to multiple airlines.

Chase Sapphire is particularly interesting to you because the miles are transferable to United. Also extremely valuable is access to the Ultimate Rewards mall, which if you funnel your spending through, you can earn 4x, 10x, or maybe more on your spending. This will let you build significantly more miles.

Amex often has transfer bonuses which are nice.

Both are a good way to top off FF accounts.