I believe that you can only install OS X PB on an IDE internal HD in the master configuration. No "slave" drive, no SCSI, no firewire, no USB.
I assume that that will change as of X 1.0.
As far as upgrading to 9, having the classic environment is pretty nice. It doesn't work all that great, but it works well enough for now. That's a pretty good reason to upgrade, but if Apple will really be including 9.1 with X 1.0, there's a good fiscal reason to hold off.
In addition, scruffy is right in that 9 is mostly a maintenance release of 8.6, and 9.1 will be a maintenance release of 9. That said, however, 9.1 will be a very nice OS in its own right. It is the last Apple OS many systems will ever see, and it is going to be the most refined, worked over (if aging) OS that we'll see in a while. (While utterly cool, I don't think X will be totally refined until a 2.0 or XI or whatever comes out. I hope to be surprised, though.)
Anyway, 9.1 is said to be much faster than 9, and have a lot of nice little productivity features and the last of the rough edges sanded off. So in addition ot being a great OS to run classic from, all indications are that 9.1 will be a very good primary OS.
Zach