I've had this debate with more people than I care to have had, and it boils down to somewhat what John is referring to.
It's key to pick one of two premises.
1) Your putting stroke remains the same regardless of weight.
If your stroke remains the same, then heavier should exert more energy into the ball.
or
2) Putter weight dictates your stroke.
If the weight of the head affects your stroke, then it is entirely possible that a lighter head would be suitable for slow greens.
But this really boils down to your putting stroke.
After saying that, I'll clarify my stance on this. I believe that a mallet with grooves puts a stronger roll on the ball than a flat blade. Especially since I've been putting much more like a pop stroke, I can see the start speed differ. So for slow greens, I'm comfortable with the spider X to give it the extra oomph off the face.