No way of really knowing for sure.
Zoro's Coa got much better hype but since he is a weapon user,the impact of an incoming attack he blocks is also mitigated by the quality of his blades. Sure his Coa made his blades strong enough to withstand the force behind his very attacks but that isn't a direct way of comparison of Coa strength between Luffy and Zoro.
Luffy Coa on the other hand was broken many times through out the new world from Hody to Katakuri but since luffy's mechanism of Coa defence is different from Zoro (weapons vs body),it is not really a proof of weaker armament on Luffy's part.
The enemies encountered are also a factor;Doffy,Cracker, Katakuri etc...all have above average to great armament haki while Zoro's opponents' hardening haki are nigh featless .
For a direct comparison,we will need Zoro to defend his body with armament and see what the result is.
For now,I think Zoro had the edge in Coa hardening pre Wano cause' much better hype .