We look for a structured program where the course of training is laid out week by week. That is, the course of study is known before hand. The more structure the better. If it can't be structured week by week, the percentage of time on each type of activity might suffice. The experiences received would add significantly to the education of the student.
Some companies have a list of concepts to be learned by the time the internship is over.
The more broad-based and richer the experience, the better. The more frequent and formal the supervision, the better. Yes, I believe that if at all possible, you should try to take all your marketing electives and not substitute one for your internship. If I were a recruiter and saw you were only taking 12 hours, I might wonder why you didn't make the most of your educational experience. If you substitute the internship for a marketing elective, you are still essentially like others in the marketing program.