That's right, just noticed that my PURPLE heavy chest is giving me slightly better spell/physical resistance than my GOLD nirnhoned chest.
The difference should be even greater with a gold reinforced.
Medium chest and All Heavy Armors benefit more from Reinforced than Nirnhoned
All other armor pieces benefit more from Nirnhoned than Reinforced.
Shields can go either way, depending on if you have the CP passive to increase their armor rating or not.
This was pretty old news honestly as it was discussed when Nirnhoned was 1st changed from a Spell Resistance % to a flat armor buff.