I'd strongly argue that a few tens of points of armour value is worth far less than a single point of resistance, particularly against harder hitting enemies some of whom can hit with tens of thousands of points of damage ~ being able to deflect 50% of that is far more than any armour value in game will manage, and the higher level armours can exceed 80% resistances against their optimum protected attacks... None of the armours has comprehensive resistance values but the witcher sets tend to do better than the majority of the 'human' armours, with minor exceptions of the Nilfgaardian and Undivk dlc sets, and they can be assembled in ways that have decent 'groups' of resistance to monster/elementa/blunt damage which normal armours are notoriously poor at overall.
There are some which give 100% resistance to poison or fire, which can be situationally useful, but especially poison resistance might be better met with high level Golden Oriole potions, so I never really experimented with these armour types.
I think the tourney is over for you and don't know for sure why you didn't get to fight ser Gregoire, maybe you were reduced to too low a health and the game considered you 'dead' in the context of the arena fight even though you were technically the last man standing. I always managed to win the fight so am not sure how it plays out if you lose. Did you have the highest score coming into the arena? It is possible that if someone else is leading then you may not be able to fight him.