I've used BoxWave screen protectors on my N810 and an Invisible Shield on my N800 and Cowon D2 back then...
The BoxWave is a lot more rigid and didn't feel resistive while dragging with the stylus, though they don't do full body shield and they probably lower the sensitivity so it's a hard choice for the next one!

Full body shield by Zagg and screen by BoxWave ?

Or maybe i should just get used to the Zagg's drag resistance but i really felt it was hard to be precise with it...

Have you tried other brands?