I install lots of cantis on Heron touring bikes. I usually recommend either the Tektro Oryx or the Shimano BR-R550. As others have mentioned. The Avids generally have a terrible squealing problem. Paul's and some of the others work fine but are fairly expensive.
Between the Tektro and Shimano brakes, both have a very similar and simple set up. Both have similar performance. However, the Tektros tend to have a bit more play in them though this seems to vary from brake to brake. The stock Tektro pad is not as nice as the stock Shimano pad. They both work fairly well, but upgrading to KoolStops is the standard course for both.
When customers are looking for the best bang-for-the-buck Heron, I usually use the Tektros. Otherwise, I go with the Shimanos. On super-fancy tourers, I often use stuff like Phil Wood hubs and King headsets but usually there is no need to get fancier than the Shimano cantis.