Which OS is best for FCE4?

Is there any advantage of Leopard 10.5 whatever over Tiger 10.4.11 when using Final Cut Express 4? I am preparing to do a very large project (about 21 hrs of analog to transfer, in sections, to digital and edit into 30 to 40 min sections for individual DVD and have wet my feet in iMovie 06, but understand that Apple dumbed down the iMovie 08 by eliminating several manual options? Thus, I am told that its time to graduate to something better and that FCE4 should be sufficient. I had problems with iMovie 06 when my segments ran over 25 to 30 min and don't want to face that issue again.