Membership program or 3 month program?

I am going to launch a group coaching program in September. My initial idea was to do a three month program, although my long term goal is to have a monthly subscription/membership model. Because of this, and for a few other reasons, I’ve been wondering if I shouldn’t launch it as a membership model instead? I just listened to Katrina’s business Q&A and she said that a membership model requires A LOT more marketing. Why is this the case? What are the advantages/disadvantages of either?