Depends on the way Oda will go with the invisible devil fruit in the Shiryu battle. So far it seems like Oda is aiming for Zoro to increase his armament haki, but later he'd fight Shiryu, who could potentially allow him to increase his observation haki too.
The way Oda portrayed it so far, it seems it's hard for even experienced fighters to detect the user(Doflamingo didn't detect when Absalom came to save Moria), so it might require for Zoro to increase his observation haki significantly in that fight, maybe allowing him to have top notch level of both armament and observation in preparation of his fight with Mihawk, who has a black blade hyped to the level higher and known as "clairvoyant" in his initial nickname - now Hawkyes - potentially hinting at high level observation haki too
On the other hand, so far Sanji showed some observation haki feats when it comes to detection, but had some limits when it came down to detecting targets swallowed by another(the shark swallowing Kinemon's torso), which seems however to be an overall observation haki limit, but regardless showing that Oda needs to focus more on Sanji's fights with his observation haki considering we don't know exactly how it impacts him in battle.
He has been caught off guard multiple times in battle without us knowing of the impact of observation, like when he didn't expected Doffy's parasite, was hit by Drake's tail in direct combat, among other things. And so far in the arc, it seems Oda focuses more on the Raid Suit, with blending speciality requiring more his opponents to actually target him and his fights not really developed towards the need of an improvement in the department(not sure with Queen's specialities though, although his zoan seems more straight forward), implying it to be developed further by Usopp/Franky, and thus giving not much development towards it, or at least not before the Shiryu battle where Zoro's observation might increase instead. Maybe later down the road though
But regardless i expect Sanji to have greater observation haki mastery overall, due to it being his speciality, with maybe Future Sight creating the difference.