Well the thing is, depends how u see it. In most cases defense missions are easier than attack IF you have 2 squads with equal skills going against eachother. If the attackers have greater skills than the defender, most likely you will go on to the final mission and maybe get the victory as well, and ofcourse if the defenders have greater skills than the attacker then you prob won't even finish the first objective.
If i see it as 2 equal teams going against eachother then the defender has a greater chance to win in most missions
Most of the times i get offensive missions too if i am playing solo because u automaticall get allianced with 2 to 3 other solo players,while if u play with 1 friend your chance of getting a defensive mission is higher(with my experience)