If having to choose a side, I find that Medea has more points against her. But what positive points could there be found about Jason?
Throughout the story we see that Medea is all along cunning and wants to get her way without considering the consequences, ie: killing her own brother, deceiving Pelia's daughters etc.... But just up to what point can we defend Jason and justify his actions?
The character of Jason in the play is negative, perhaps because in Greek tragedies the hero must have some kind of "tragic flaw." Jason's flaw might be greed or love of power that marriage to the princess of Corinth would bring.
The Jason of myth is very heroic. His uncle had stolen the throne from Jason's father, and when Jason came of age, he set out to reclaim it. But his uncle made him prove himself worthy by finding and bringing back the Golden Fleece. Along the way, Jason freed a king who was harrassed by harpies, he harnessed two wild bulls and plowed a field; then he had to fight off the army of soldiers that sprang up from the seeds he sowed in the field.
Also, in the myth, Medea does not fall in love with Jason on her own but is enchanted by Aphrodite to love him at first sight.