Personally, I will avoid screen protectors and just keep my N900 in a soft cloth case like I did my Xperia X1.

The stylus should NOT scratch the screen, unless you are really careless and jab the screen like you are trying to murder someone.

The ONLY time I ever scratched a screen was when I accidentally put my keys in the same pocket. Since I learnt from that mistake and got the soft cloth case (and am more careful which pocket things are kept in) I haven't had any problems.

If you are worried about dropping your N900 then a screen protected is the least of your worries anyway. You are probably more likely to break something important like the glass of the LCD itself, the case, the keyboard, etc. I know the touch screen would be the LEAST of my worries if I dropped it.