Both men have flaunted their abilities at various stages of this manga and what's clear is that they could both kill each other, in one way or another.
Thinking through their mental states:
Crocodile was an up and coming hot shot and was stomped on by WB. This left serious emotional baggage through the rest of his life, where he lived in denial of it, trying to cover it all up with a coating of mafia badassness. Having left the past behind he moved onto bigger and better things. With his confidence in one hand and his DF in the other, he faced off against Luffy as if he were little more than an annoying bug. Crocodile taunted his victims, made them feel pain and left them to die in horrible ways. His arrogance was beyond anything (having forgotten his shame of defeat against WD). In round 3 vs Luffy he was taken by several shocking events of Luffy's undying resistance, will and resolve to win. Having nowhere to run to he succumbed to fear and was left helpless against a raging Luffy. Since then, his time in ID let to his acceptance of defeat. He was able to remember his life as it was and when the opportunity came, his resolve to redeem himself and fight once more came to life. Even in the face of Luffy, he felt acceptance and never once challenged him or had any animosity towards him over the passed. His head held mid high, with clearer sights on life, he wasn't going to be a guy to lose again so easily. He has learnt the humility of defeat and moved on from it. He knows his weakness, he always has and now he is even more deadly than ever. He won't be making the same mistakes twice. He is smart and devious and will do anything to win.
Rob Lucci... I feel less passion for him, and we all know a lot about him. Kid with issues brought up under the wing of the government as a killing machine with no regard for anyone's life. Militant and extreme until his epic bout with Luffy. What did he learn? Perhaps the government wasn't so good. Maybe not all pirates are bad. Maybe he learn't something about humanity and compassion. Maybe he got soft? Or did this make him even stronger? Either way, he had no weakness. He was simply matched and beaten strength for strength. Whether he took some time out with the rest of the gang to help others, do some good and find peace or trained the hell hard to get stronger is unclear. I like to think the former.
The way I see this fight is a square off at the start. Both go in heavy and full on. Lucci probably knows Crocodiles weakness.. hell if he can do anything about it off the starting line. Crocodile knows Lucci probably knows his weakness. (Hell, Crocodile can assume everyone knows his weakness.) Regardless they attack. Crocodile gets some contact with Lucci and drains him of bodily fluids. Lucci in a reduced state is left struggling to find footing. Crocodile toys with him until seen fit. Poisons him and leaves him to die... scratch that... he sends him to Davy Jones. No survivors.
