I think the additional mass will ultimately prove to make the thick fin HS better overall though.
…and right there you're off into the woods with just how complex something that appears simple can actually be.
1. Mass in and of itself is
not helpful with thermal efficiency - what counts is thermal
conductivity - how fast heat will move through it.
2. The "thin" sink assembly, while it may appear to be less "substantial" than the heavy cast one, actually has more surface area to pass the heat off into the air.
3. The biggest "problem" area on all of these is the immediate vicinity of the CPU's. That's where the heat is and what counts is how fast it gets carried away from the chips before spreading out into the sink and finally dissipated into the air. This is what makes the copper model superior with its gas tube. It's very good at preventing a "hot spot" right above the chips.
Like I said, they all do a pretty good job and while we can endlessly debate thermal dynamics, at the end of the day it's all academic.