Well, of course I am going to say the iBook is BETTER, but from what you do, it doesn't sound like you are going to gain a great advantage by buying the iBook. I don't know if you are using OS X, b/c if so, then you are going to see moderate improvements over the Pismo, but those improvements are all going to be interface improvements. If you are using OS 9, then there is no real difference in the speed of the OS itself.
Think of it this way: Do you wait long for the computer to do what you want it to do? Would it make a ton of difference to you if your applications booted up twice as fast?
Remember, the Pismo was designed to be the top of the line. Those top of the line computers can last a LONG time before they need to be replaced, and it seems you have offset some of the problems with a ton of RAM. If you are using OS9, then just make sure to keep Virtual Memory off. Lol, I still can use a PowerMac 6500 with OS9.1 fairly well. I just maxed out the RAM and for the most part, it can keep up with anything that doesn't require a G3 processor. We are talking a 250Mhz machine that was the top of the line back in 1997, and it still works for the basic tasks that you are talking about. So personally, I would hold off as long as possible. Plus, the Pismo is more expandable than the iBook (as in you have swappable drives and can have up to two batteries in it at once).
Just my four cents worth😀