Depends. Do you have Bossk's lead zeta'd? What about his unique?
If you are asking if tenacity is better than potency (or vice versa) under a bossk lead, it doesn't really matter for Bossk. You get healed whether the debuff lands or gets resisted.
If you have Bossk's unique zeta'd and he's going to go first (whether it's under his lead or Jango's), it's probably best to have him with as much tenacity as possible. Even if he doesn't go first and JTR will be able to get a wave off on him, it's still better to resist the rest of the debuffs on that ability and be able to have Bossk taunt or do his group hug.