Its a tough call. The monochrome m500 has better visibility overall in all lighting conditions. The m515 has color which is nice. More memory and is still good without backlight in good lighting conditions. But it needs the backlight on in more situations and thus has shorter battery life.
@kelbot @requiem @Jetengineweasel I would prefer the left one (better battery life, better outdoors visibility - and I don't think that the early color screens are good - I have the Treo 90 and my eyes are not very compatible with the screen backlight).
But my view can be biased because I am the active Palm III (TRGpro) user ;-)
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