I realize there's a school of thought that says Lab should be pronounced L-a-b. (And possibly, that's how the creators said it.) Problem is, unlike the letters in RGB and CMYK, the a and b in Lab don't stand for anything. Only the L is an acronym. And my research into the origins of the mode suggest that it's no coincidence that the channels' initials L-a-b spell out lab, as in laboratory.
My core argument, "Lab" is how just about everyone says it, may not sound particularly convincing. But consider, CIE designated Photoshop's variety of Lab as CIEL*a*b* D50, which is not how we write or say it, so whatever choice we make is an innovation. Words evolve, and I regard the one-word variety of Lab as a positive evolution.