One thing to remember (which I only bring up because it annoys me so :biggrin is that you can't tell underwater buoyancy from dry weight alone!
Compare the size of the second stages, of course, but don't get hung up in their dry weight. A second stage that weighs slightly more but displaces more water may well be "lighter" when underwater than another second stage that weighs less but also displaces less.
(For second stages, compactness and underwater weight tend to be in opposition. Thankfully, there don't seem to be many weight-of-a-brick regs around, so find one that fits and use a comfortable mouthpiece -- ComfoBite, Atomic, SeaCure, whatever -- and the in-water, in-use weight of the second stage shouldn't even be a factor.)
Compare the size of the second stages, of course, but don't get hung up in their dry weight. A second stage that weighs slightly more but displaces more water may well be "lighter" when underwater than another second stage that weighs less but also displaces less.
(For second stages, compactness and underwater weight tend to be in opposition. Thankfully, there don't seem to be many weight-of-a-brick regs around, so find one that fits and use a comfortable mouthpiece -- ComfoBite, Atomic, SeaCure, whatever -- and the in-water, in-use weight of the second stage shouldn't even be a factor.)