While I typically prefer Windows, Openfire is much better tuned to Linux. The cache in the Windows version didn't manage itself well and crashed often. If you have a small install, it may not matter much. We fought with the Windows install for a long time and decided it wasn't workable. Openfire ran without a hitch on CentOS 5. Once you have it installed, you need to do little with Linux. You can just manage the app itself with the same web interface you already know.
Edit: Was originally installed on Windows 2008 R2. Worked fine until about 100 concurrent users then no amount of cache increases seemed to help, just bought a little time. Might be better on a 32bit Windows OS?