Well, some things to ponder . . . .
Zire 21 only has 8MB of memory, a 16MHz processor, and no ability to add additional memory.
Palm makes ya' learn their grafetti in order to write in your messages. You can use the key pad, but that tends to be a pain.
Using the word processor program means that you may lose formating, and you'll have minimal fonts to work with.
The $100 limit only give you the 2 choices and you may find that neither fit the bill you're looking to fill.
Being the odd ball that I am, I went with the Sharp Zaurus when it was on Clearance at Staples and only paid $139 for it. It runs Linux, but has 64MB of RAM and a 206MHz processor. The Hancom Word and Spreadsheet look identical to what you find on your full size computer and you have many of the same formating, fonts, and formula's that you are used to. Plus, the hand writing recognition is good and easy.
Just a couple things to think about.
Harder