If money is no object, general consensus seems to be to go for the 15" with SuperDrive. Faster, better graphics, etc. The 17" is generally seen as too big, but many would disagree with that.
The big plus to the iBook is that the case is more durable (and, of course, it's cheaper, but that doesn't seem to be an issue).
Best thing to do is to go to an Apple Store or other reseller and play with them.
If money matters somewhat and you're looking for a small, really portable one, general consensus is that the 12" iBook is a better deal than the 12" PowerBook.
Again, though, any one of the models has those who emphatically state it's the best and those who don't like it. Go hold them yourself!