well, I think you guys have summed it up -

in lab conditions ie. no wind whatsoever (or just a steady consistent drift) then there is no argument - lighter will fly better, smoother, slower, more graceful and more responsive, not to mention better vertical

where gusts, buffetting and cross winds come into play - well, the super light plane will be buffetted about like a kite on a string in a storm, while the more massive model calmly handle those same conditions

It's the same reason slope planes and gliders have ballast - on windy days you need it or you'll get blown away, literally.
On calh days however, the last thing you want as a glider pilot is dead weight, as your goal is to exploit the lift and stay aloft sa long as possible.

There is no "right" plane. A heavy plane in perfect conditions will look clumsy and lethargic in comparrison to a light one. If the wind picks up, you'd better land the light one ....fast.

Moral of the story. Build as light as you can, and include a ballast bay - adding weight is very easy - removing it is next to impossible.

I.F.