From what I understand about modems, just about any serial modem will work with linux and just about any serial modem that will work with linux will work with windoze. I haven't personally tried those two modems, but linux is flexible. (Heck, I'm online with a linmodem!)
Also keep in mind that just about every piece of hardware you buy will have drivers specific for windoze, so I wouldn't worry about incompatability there.
good luck in your search for a cheap modem
