Mirago may result in the least exposure, but can be a cheap way of getting a number one placement and showing of a good "title sentence" on results such as ask Jeeves.
Few of the Larger commercial sites have bid with Mirago, but with a minimum 10p bid, I think they don't stand much chance in this market unless they can make some significant alliances with feeds to the major ISP's
The Espotting feed to Yahoo is interesting but I am yet to be convinced how much UK business this will generate. We are watching our Espotting costs closely and have yet to see a significant increase since the feed started.