What is the best way to mirror data across 2 load balanced servers? Is there a way better than rsync or unison? What about with coda or NFS?

It depends on how frequently you need it updated. We have a few customers who use this and have rsync setup, it seems to work pretty good

I need something more frequent than rsync. I know you can set it to rsync every minute but with a lot of files seems like it would overload.

Would codafs be appropriate? what about NFS?

If I have 100gb of data is it even practical to use rsync or unison? Wouldn't it take longer than 1 hour for rsync to read through that many files to determine if it was out of date?