While Zoro pretty much is only a fighting force, without anything else bringing into the crew (which seems to be enough for alot of people), would Killer be way more usefull. He would analyze the enemies pretty fast, would bring them down lethaly and fast, wouldn’t get lost all the time...
Killer is kinda the strategic force Luffy is needing in his fights. With him, telling him some weaknesses or warning Luffy in time, Luffy would have way less problems in paradise and i think even after. Yes, Zoro is strong, but beside that, what does he have to offer to the strawhats, compared to Killer?