Word 5.1 was an excellent product, and Word 2001 is about as good (so long as you turn off all the irritating "helpers"). Word 2001 has, over the years, been rock solid under OS9 on my Wallstreet, and ran more stably for me there than even 5.1 did on a 68030 chip, where i used it for many years. As for Word 5.1 under OS9 - well, basically I would not really consider running 5.1 on a ppc to be a good idea, because the code is all and without exception 68k code.
I have recently begun to use NisusWriter 5 on 68k hardware, and find it a much more versatile and able tool than either of the MS products mentioned. Nisus looks very plain-Jane, but it is a true writer's tool. One of its best technical features is that it does not demand that the machine makes constant disk access, as Word in all shapes and varieties does, so that it is nicely suited to running on a powerbook (so long as that powerbook can accept a reasonably large ram complement - hence I run it on a Duo).