I sourced the CD150 diaphrams from a place called: www.classicautospares.us
Their price was $5.00 each plus shipping.
As for response, a beefier diaphram will respond slightly slower to vacuum changes. If you are a racer, or die-hard originalist, that might be an issue. I would guess that durability would win out over barely perceptable changes in throttle response.
But that is, of course, a personal issue. "Better" is a loaded term.
I do not know if Mercedes used Stromberg CD150/175 carbs or not.
Keep the rubber side down