Since no one else has piped in yet I'll give you my 2 cents, at least on the Kessler.
It's 50% plastic and 50% HR
Medium face
~0.100" opening
Straight wall chamber
Based on vintage metal Otto Link
This is my first and only tenor mouthpiece, so far any way, so I can't vouch for any performance comparison against any other mouthpiece but I've been very pleased it it so far. Kinda wish now that I had upgraded to the Kessler OL7 Pro but at the time, all my experience had been with an alto and I wasn't sure how I would adapt to a mouthpiece as open as this one. So I didn't opt to shell out the additional bucks but it been far so good!
Sorry, can't help on the Selmer.