What makes it lighter? More bubbles in the sauce?
I'd be curious to know if the fire rating is lower as a result.
But flexibility with drywall is the LAST thing I'd want, unless I'm building something like decorative arches over doorways.
But other than that, I'd prefer the most rigid product I can find to span those sixteen inches.
I certainly don't want to see waves running down my walls or ceilings after a few years.
Bob