I've been looking at these too, both seem to provide fully functional XMPP servers in Java. I know Tigase is designed in a very modular way, not looked at Openfire in as much detail yet.
My intended use would be to create a custom IM-based app, using XMPP for convenience rather than to open my server up to talk to other XMPP servers.
I'm trying to evaluate my needs based on the following, roughly in order of importance:
- Documentation coverage & community
- How easy to plug in own functionality
- Licensing/cost - I don't plan to release my code
- Maturity and stability