My view: The main issue is not so much climbing as centering. As others have noticed, around 2-3 knots if you're flying with a gaggle that's doing all the work of finding thermals, water vs. no water is about a wash. It feels bad to watch them glide past you, but you do gain it back again in the climbs.

The real issue comes when you're out there alone, low, and you have to center a punchy thermal. It can literally be impossible to climb. You flail around in the nasty bits and pieces watching the fleet go by overhead. Eventually you give in and dump the water and now magically you climb at 5 knots.

Flying with water you're going faster, so thermal circles are larger, and you're less maneuverable.

So really, yes, the main consideration is how punchy the thermals will be and how often you're going to have to find and center one on your own.

Many of my if Idas are memories of great days blown by getting low and not dropping the water fast enough

John Cochrane BB