For those concerned about their credit rating, I think the 20k for $60 offer makes better sense. Even if you look at it that you are paying $60 for 5k more miles, that's a very good rate. Since there is realistically a limit on how many cards you can apply for (I apply for a new one every year, I'm not sure I'd do it more often), you are limited on how many cards you can get (bonuses you can receive). It would therefore would seem to make sense to maximize what you are getting with each card. This is even more true since the 20k offer seems to be an abberation that at any time could get pulled for the more widely advertised 15k offer.
Jeff