Well, the bus is the same on both machines, well arguable better on the iBook.
The chip cache is slightly smaller on the iBook.
I would say that the iBook would be more than comparable!... although only time time will tell for sure.
The new iBooks are great machines. I bought an 867 Tibook a while back for over £1400... If the iBook had been available with a G4 at the time, I would have ticked the optional bluetooth box, and be using it right now... with the killer feature of having spent only £750!!
It is a great, great little machine.
Peace,
Marc
P.S. Remember that the iBook will also have Airport Extreme which the Ti won't...