I think David Leys is right. Find the setting (i.e., the "gear") that allows you to maximize *your* power output, and then crank like hell.
For me (short legs, bw 160) that seems to be just about the middle of the range . . . too "heavy" and I get a lot of power per stroke but my stroke rate falls too much, too "light" and I can stroke at a high rate (35-40/min . . . well, that's a high rate for me, anyway) but each stroke doesn't contribute enough wattage.
Generally, comparing among other users of the machine, the bigger and heavier you are, the more you'll prefer the "heavy" setting that makes the most of each individual pull, and vice versa.
As always, YMMV, each cat his own rat, etc.