If you pay off the account, you will get hit with fees of ~15$ month for 12 months. = 180$ to get 20k miles = about 1c/mile
You can get the 100% bonus from Delta and get miles for 1.2c right away. This promo will take 1 year to get the miles and the amount is not very big.
First 10 k comes quickly, the next 10k will take 9 more months!

If you keep 6k in the account, you are taking the money from elsewhere.
You could have taken that money and paid off your mortgage or home line of credit with the money for 1 year; at 5%, that comes to 300$ in charges for about 20k miles - true cost is 1.5c / mile, after tax cost about 1 c / mile again.

The more I research this, I think it is better to look for deals from either US Air or Delta and get a whole bunch of miles right away and use them up for international travel.
But if you have an account with Citi already and are willing to jump through hoops (and which FTer is not?) you will get a small reward.

I personally got more miles applying for a Citi Visa, MC and Amex. I have become tired of churning and keeping up with the statements and bills due.