I prefer Zoro as a character. Think it is because i liked him since the start of the manga, he is part of the main group and was there in many epic parts of the story.
I cant deny that Law FB was, together with Robin, one of the most tragic, and he have gotten a lvl of development that not even some SH had, expecially post TS, mailing him the most revelante character in the manga besides the East Blue 5 and Robin, but still can like him. Pehaps is because he and Kid were writed to be seen as paralels to Luffy, what is hard.
Design wise, both arent that unique, as we can see a bunch of guys in manga like Law, this smart-tattoed guy that the fujoshi love, and Zoro, his desing, like the other East Blue core members, is a more simple one, but i find it very effective how Oda made those desing become iconic, and the third sword give something more for him. Of course he is suffering from a lack of drip in post-TS, as i always find more cool whe he isnt using japanese kimono, as it make him look like the generical manga samurai, while when Oda draw him in other cloths i think that the contrast between the japanese swords and the western clothing improves his drip (like in Dressrosa, Skypea and Alabasta), hope next arc Oda let all Zoro Kimono in Wano.
About the character Arc, i have to agree, thats a thing all the SH have been suffering in post TS, where the arcs have been focused more on other side-characters, especially Law and the Wano folk. Zoro only got more time to shine here in Wano.