The hospital grade pumps are much quieter, which does not matter for hospital or home use but may make a difference for the workplace.  The motor is more powerful and more durable (since it's designed for multiple users who each buy their own set of tubing) and also achieves stronger suction.  

I don't know if any studies have been done, but I briefly rented a Medela Lactina with my first child and was immediately able to express about a half ounce more per feeding than I had been with my Medela PIS.  Over the course of six pumpings at work and home, it added up and helped keep her formula-free.  The IBCLC I rented from told me that a regular double-electric is best used to maintain supply, whereas a hospital grade is better for increasing supply.

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