Summer internships are not a requirement of the program. The only advantage to setting the internship up as a credit internship is to have it appear on your transcript. It is advised that you seek the assistance of your faculty adviser or program director in helping to structure the internship to gain the most from it. No, there is not a requirement for a cooperative or internship experience. However, it is strongly encouraged.
Experience is THE differentiating item when employers interview candidates for full time employment.
Employers are basically demanding it. You would be well served to have several intern experiences at a minimum. The decision is preferential from your point of view since this is not a program requirement. If you can afford taking a semester off from formal coursework, you would probably have a fuller, more enriching experience with a coop. The coops are 6 months in length and allow for longer term project work than a short, less than 3 month, internship.