Regular workshops are more flexible and is more ideal for the typical participant who is looking to learn a new skill, find something fun thing to do with friends, or to gain quick knowledge about a topic. A variety of participants will be expected to attend and are not expected to attend frequently.
A multi-week session is ideal for workshops that need more than one class to teach your topic. Participants would be expected to attend each class in the session in order to fully take advantage of the topic you are teaching. This option may be less flexible for participants, but would be an ideal format if you are looking to get to know participants and build community.