
> I'd read that the traditional positive face is easier to read in direct sunlight
I'd probably agree with that, which may seem to contradict my comment above... It may be due to the difference in illumination levels; where there's bright sunlight, you get better contrast between a 'white' background and dark figures, but if you only have a feeble light source (the backlight), the contrast is better if it's light figures on a dark background.
I really ought to ask a friend of mine who spent a number of years researching emergency signage, illuminated and otherwise; I'm sure he'd know. It's noteable that road signs are dark figures on light backgrounds. Oh, except for the ones that aren't (blue and green backgrounds)...
Hell, get one that looks nice...