This isn't about warmth, but I like the stiffer sole on Hyperflex boots. I am far less likely to get the dreaded toe roll on those. I'm currently wearing 7mm Billabong Furnace Ultras. Very warm, though I got a toe roll on my very first wave with them and have never forgotten it.
For the toe roll I got some of the Solite booties. I have to re heat mold them as they shrunk over the summer. Also- I haven’t lost the little socks that come with them. Pretty proud of myself for that!