I'm an intermediate level kayaker but have only paddled plastic boats. I'm looking for a kayak to do multi-day trips in on the ocean. What are the differences between plastic and fiberglass?
The differences between a "roto-molded" plastic and a hand made fiberglass boat are quite substantial.
If you are looking at a high end plastic boat from the likes of Current Designs, Nigel Dennis, Valley, or similar you will likely be happy with the performance unless you are a very high end paddler. Many of the lower priced plastic boats however don't perform well in rougher conditions.
If you are a true performance boater you probably already know that you want a fiberglass boat. Just be prepared to take care of it, and it will serve you well for a long time.