Jack still has one very specific thing going for him: he's not afraid to use the environment or otherwise "underhanded" tactics to overcome his enemy. Countless times in his series, we see Jack take on armies of robots using more than just his sword. He makes all manner of different traps, at one point he fought a ninja by their own stealth techniques against them. From what I've seen of One Piece (not much admittedly) Zoro is practically the opposite, falling squarely in the "Honor Before Reason" camp and essentially powering his way through fights using his own sheer force of willpower and swordsmanship. Whether he's "stronger" than Jack is not the only concern, it's whether or not he can outwit Jack and... well from what I remember about him I don't see Zoro as that kind of fighter.