Dave,
I have been to both, sometimes in the same year. FedUC is a bit shorter than the international conference, and has fewer topics. It has more going on than the typical state GIS conference, but only due to the large number of federal agencies that are able to attend without a registration fee or much travel. It could potentially be useful to someone not able to attend San Diego. That being said, both the developer conference and international conference are well worth the trouble and expense. UC has an average attendance of 10,000 people, and there is nothing like it. (Well, maybe FOSS and Autodesk University, but still.) Everyone who uses ESRI should go once.
I suggest reading the agenda, and that will answer whether you ought to attend any particular conference.
Jay