Zoro has more power imo but Sanji has more precision and control. Zoro didn't do the feat of cutting and not-cutting in a single slice until after the timeskip when he cut the shackles off the merman royalty. Sanji was pulling kyoshiro cutting komurasaki's blood bag but not her body level swordsmanship back in enies lobby. Zoro cuts the whole train in half in the same chapter Sanji's cutting noodles so its actually a good show of their different specialties