I rented the DUI Powerstretch 300 fleece, and then I ended up buying the 2 layer arctic fleece from Ron.
The DUI is thinner, but denser.
I'd estimate that the double layer fleece Ron-gear is probably at least 1.5x as thick as the DUI 300 (which is also double layer) in the air. But it's softer and compresses easier. I found I had to put a little more air in the suit to combat squeeze with the Ron-gear than with the DUI. I was warm in both, even though I run cold.
I express no opinion about how much extra weight the ron-gear undies might require. I'm still working out my drysuit weighting.
I like my ebay Ron-gear undergarments, and will be keeping them!
As far as fit, to add another data point to the thread, I'm female, 5'7", slim. The size medium is perfect height-wise, or maybe a little too tall. It's quite big around though, lots and lots of room for me to grow . . .uh . . . in circumference! Like somebody said above though, doesn't appear to interfere with usage, although my drysuit legs are a little tighter to get on now, and the Ron-gear fleece doesn't slide in quite as easily as the DUI.
Hope that helps.
Taxgeek