This predicament is one of the reasons why I created InternshipRatings.com. While it is important to consider the skills I will learn and the tasks I will perform, the company environment and staff play a large part in the success of an internship experience. If the companies I am interested in were posted on the website, it would make my decision much easier! Unfortunately, I do not know anyone who has interned at these companies and therefore must try to make the most informed decision I can on my own.
As I sit here weighing my options, I will offer you some advice if you are in the same position. Ask for the company's internship program fact sheet/guide. This document should clearly define your summer duties and the company's expectations of you. The one I received even details how to answer the phone properly! As I have also mentioned before, the interview is also a great way to get to know more about your potential employer. Thoughtful questions can provide you with similar answers that a previous intern would give.
If you have taken this advice and feel confident about both internships, you should feel confident selecting either option. And when you are finished with your experience, don't forget to add it to InternshipRatings.com to make a fellow intern's next decision an easier one!