So I ended up getting a Greenprene 7mm, and so far I like it.
I mentioned upthread that my Thermaxx had pulled apart a little at the lower back after about 100 dives. I took it in to the dive shop just before everything shut down to see if it could be repaired, and they called me a few weeks later to say Henderson would refund half the purchase price as it was still under warranty. I thought that was pretty fair and decided to stick with the Henderson brand. (It also wasn't a great time to be trying on a bunch of wetsuits, but I knew what size would fit me best in that brand, even if it wasn't a perfect fit.)
So I got the Greenprene, and I've worn it twice now; once just for a swim/snorkel in the ocean, and once on a legit dive this past weekend. I thought it was maybe just a tiny bit warmer than the Thermaxx, though maybe that's because it's brand new. It did seem a bit thicker around the wrists; my Teric strap almost wouldn't buckle around it. But my weighting needs didn't change, so it seems it's equally buoyant. I thought it seemed just a little roomier inside, like it ran a bit larger (sadly, I can definitively rule out the possibly that I lost weight) but I didn't notice any cold water flushing in. It was still stretchy and easy to get on and off, which is my top priority. We'll see how it stands up over time to my apparently colossal butt.