It depends on the contract whether or not you're eligible to choose miles. The reason that miles are excluded on many contracts is that Nationl charges something like $0.50 a day for the miles which makes it equivalent to you buying the miles for $0.01 per mile. Most of us in the forum choose National rental credits since the value is much higher.

Look at it this way, even with 7 credits for a free rental day - you'd pay $14.00 for 1,400 miles or you get a free day rental (value probably $20.00 on the low end - or up to about $100.00 on the high end mid week one day / one way rental).