David's response is pretty good (as is Minh's Wink Here's how I think about it:

If you are looking for a conference that will give you immediately practical training, so that you're more productive the minute you return back to the office, then TechEd is for.  Whether you're a developer or in IT, TechEd will have sessions and labs focused on the technologies you rely on every day.

PDC, on the other hand, is where we talk about our strategic platform roadmap for developers and architects -- what are the attributes of the next generation of applications, and how is the Microsoft platform growing to enable those kinds of apps?  You should come home from PDC with a clear understanding of what Microsoft thinks are the signficant trends in software, and how you can architect your own procuts over the next few yeras to take advantage of them.

If you're a person providing input into the 2 to 3 year roadmap for your products/services, PDC is for you.