IMHO the credit card miles are not a good deal anymore. They used to be when I was a frequent flyer, but not so now. IIRC, the rule of thumb is that you need to earn 2.5 miles per dollar spent, but most plans only give 1 mile per dollar. Plus there is the fee. If you can find a miles card with no fee, then there is no risk, but they are a rare bird.
I would just get a cash back card with no fee (I use Quicksilver from Capital One which give a $1 back per $100 spent), and put the cash back in a special bucket for future airfare.
The other problem with FF miles is that they are difficult to use, as the airlines only have a handful of seats on each flight, so it can be difficult to get the flight you want unless you book well in advance. I'm meticulous and planned down to the exact date and time that worked the best for me, so I was always using double miles to get want I wanted (no restrictions on double miles usage, and they put you in first class, at least on American. Not sure if this loophole is available anymore.
HTH. There are websites dedicated to gaming frequent flyer programs, which probably give better advice than me, but it is too much work, at least for me, to do all the things those folks do.
