I am currently trying to decide among various online services for hosting and selling photos, and these two seem to be the most popular.
In this post, I link up to discussions that I think are useful for comparing and deciding between Smugmug and Zenfolio.
Pros and cons
This thread has some interesting notes on the advantages, drawbacks and features offered. John replies here on the new features introduced by Smugmug in July 2006 for the Pro subscribers.