If I were given the choice and all I did was word processing and surfing, I'd go w/ the M. I'd also put Win2K on it and not XP.
But, if you can find a P4 w/ what you want and at the price, get it. More bang for the buck is what you'd have.
I'm a big fan of Mwave.com. You can find some . . .
HP/Compaq IBM Sony
. . . all in the range you're looking at. If I got to choose, I'd go w/ the brands in the order I've listed them.