Linear is easier. Branching augments the complexity a great deal. If you're not up to the task of writing several branches twice, or more times, to include an history that can be really modified according to the actions you take, it's better to have a rich linear history, were every mission advances the plot a little, that have a branching campaign plagued with meaningless decisions, that only change the cinematic immediatly after the mission, and what unit you get from completing it.