I know this is not exactly an answer to your question, but if you are switching out the panel, why stick with XFCE in the first place?
I only ask because I myself used XFCE on Slackware for about 2 years, and while I was totally happy with it from a design point of view, it eventually became obvious to me that their must be a massive memory leak in it. After a few days uptime, I would always have to kill X to prevent my RAM from being totally devoured.
I have since switched over to E17, which I not only like better than XFCE, but is so far much more efficient with system resources.
That said, fbpanel is very nice, and I have never had any problems with it before. It should integrate perfectly into XFCE, and (this is just my gut feeling here) is probably going to be a bit faster than PyPanel, simply because it is written in C.