Overall, character, strength treatment in general
		
		
	 
 I like Sanji more overall, but not by a huge amount. Both are some of my favorite characters from the manga after all.
In terms of character, Oda gives more attention to Sanji, but this does not inherently mean Sanji is more well written or that Oda cares more about him. It's more that Sanji is simply a more tangible and fluid character, while Zoro is more stoic and stagnant. It's hard to write an arc like WCI for Zoro as he simply has a different story from Sanji, he's got a different personality, he goes about things differently than Sanji. This is why, whenever Oda decides to give good character moments to Zoro, he opts not to dive into his past, but he tries to have Zoro build off current issues and dilemmas (as seen in arcs like Water 7 and Thriller Bark). He makes Zoro's changes a little more subtle than Sanji's, but they're still fairly evident. We see him building trust and love for the crew ever since he joins it, starting off as a character that seems to mostly be focused on improving his strength and drive so that he can eventually surpass Mihawk, only to eventually switch to a character that, while still heavily focused on improving his strength, would be willing to give up the dream he had been dedicating his entire life to for years on end for any member of the crew.
He's also more similar to Sanji than some people would think, the only difference is their attitudes. Like how both seem to believe that women are weaker/softer than men are, but while Sanji would take that to an extreme and decide to NEVER hit a woman and ALWAYS forgive them, Zoro is more logical about it and still hurts women whenever he needs to but goes easier on them than he would with a man (look at how hard he butchers people like Lucci yet he couldn't even bring himself to actually hit Monet, he just paralyzed her with fear).
Though this is mostly due to the fact that, for Sanji, before Zeff, the only people that ever cared for him were women, and a woman also helped him escape Germa and start his own, more desirable life, iirc. And even Zeff emphasized that he should be kinder to women than men.