I compare them to the disparity between oil and water.
Wesley taught that man has free will and must
choose to believe in God. He believed that something called "prevenient grace" somehow tames our sinful nature to the point where we're able to freely make a choice between disbelief and belief. He did not think that God's grace is effectual. Instead, man has to cooperate with grace to be saved. Wesley taught that regeneration happens after salvation. He also taught that you can lose your salvation.
Calvin taught that salvation is wholly by the will of God and that grace is irresistable. God, by way of his grace, regenerates the hearts of the elect and gives them faith in Christ as a free gift because he loves them. Calvin rejects the notion that men can choose their own faith because men are completely embonded to sin--all they desire is sin. Wesley thought this too, but believed in the aforementioned "prevenient grace." Calvin repudiates this notion, saying that the grace of God always accomplishes its purpose, which is the regeneration and salvation of the elect. Calvin also taught that once a believer is regenerated, he can never lose his salvation because the Lord will not allow him to.
Soli Deo Gloria
Jon