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:
Do not use Openfire if you expect to scale beyond a couple of thousand concurrent connections.
Tigase is amazing at handling hundreds of thousands concurrent connections and is wonderfully architected for largely distributed platforms where XMPP is simply the external interface. It comes with a price of rather poor documentation. You often need to go and read the source code to understand what's going on.
Openfire is perfect for small setups and its API is simple and very well documented. Unfortunately, it's not architected to scale even nearly close to what tigase is capable of.