The ones I got on following flights had buttons all the way through, not only at the top as you mentioned. Can't say whether it's lighter or not since I never had any of those ones with only buttons at the top. I'd say the top is substantially thicker than in comparison with a normal T-shirt.

Beginning of August JFK-HKG: Black with Green piping.
Beginning and middle of October HKG-SFO-HKG: Black with Pink/Red piping.

But agree with others that I had as well difficulties sleeping comfortable in those with blanket and in those cabin temps. However, I do get into the pajama pants to keep my other stuff wrinkle-free.