A brief search seemed to indicate that Eclipse has more (and more up-to-date) plugins and tends to be supported earlier than IntelliJ (if the latter gets support at all).-ewemoa
How
many plugins do you need - what kind of plugin
quality do you need?

Core-functionality-wise I got the impression that they aren't all that different from each other with more comments about IntelliJ being less confusing to use than Eclipse (may be it's just a matter of what one is used to...).-ewemoa
What I hear from the coworkers that praise IntelliJ is that it's "more powerful" - stuff like better refactoring support, and tooling for various frameworks (the latter probably mostly important if you work on larger/commercial projects). Also, (at the expense of somewhat slow startup time?) it's supposedly a fair bit faster than Eclipse at doing refactorings, project-wide searches (not just full-text search but semantic searches), et cetera.
On a non-Java note, I'm looking into Scala and noticed that IntelliJ has some kind of support and there's this plugin(?) for Eclipse: http://scala-ide.org/ -- has any one had experience and opinions on the latter?-ewemoa
My general impression is that Scala tooling is still pretty rought around the edges in general. I took the Coursera
Functional Programming Principles in Scala course, using Eclipse. It was a major PITA to get the right plugin versions set up - nightly vs. semistable channels, and different versions (for different versions of Scala) added to that mix. Also, I had some weird fluke-outs where Eclipse wouldn't recognize scala files, or would throw very weird errors, or where the Scala Worksheet would mess up and be wonky.
The Scala Worksheet is
very cool though, it's like a REPL on steroids. Useful while learning, and beyond that too.
I see that the most recent version of IntelliJ has
support for the
Play 2 framework. The Eclipse support for Views was basically non-existing, at least as of a couple of months ago.