I think your points are excellent. The decision isn't really whether or not to upgrade; rather, to upgrade now or later.
I think you have built a good argument to upgrade later. Why do I say that?
1) The speed, if anything, is a bit slower on Mac OS X right now -- especially on older machines. But that will change.
2) You don't have AirPort.
3) Chances are your hard drive is too small to partition it to maintain two complete OSs and their associated apps, and you probably don't want to invest in a new drive for an older machine -- no dis on your Lombard; they are great!
4) Most likely you would spend most of your time in the OS 9 partition anyway, based on how you described the way you use your PowerBook.
So, looks to me like your decision to sit tight for a bit is a good one. It will be harder to do so, however, later this year when we see more refined versions of Mac OS X and lots more apps to run on it.
You might want to start budgeting now ...