Heheh, IMO, Shadow Mask is suck! Aperture Grille(Trinintron/Diamondtron) is superior. Those lines are your assurance of quality!
It's all personal preference, but for me I'll never choose Shadow Mask over Aperture Grille. In fact my first Aperture Grille was a Gateway 15" monitor! I loved it up until the day it died, but replacing it with a 19" Mitsubishi Diamond Plus 91 has eased my pain somewhat.
My advice: If you haven't really noticed the difference between SM and AG, go to a computer store and check out the differences there.