I did Jillian Michaels' 1hr strength and cardio programs on Medifast. I noticed no problems. I found it easy to follow, was never hungry and I lost constantly, though the losses were slower than on IP. I switched to an IP-like diet since. I found you cannot exercise on IP for a couple of weeks until your body readjusts. It is lower total carbs than Medifast and maybe puts you in ketosis whereas Medifast is borderline for some people. I still exercise, though I switched to Body Revolution, which is a shorter workout. I currently eat more than IP recommends, usually 4-5 packaged meals depending on whether I work out once or twice a day.
Overall, Medifast was easier to do for me as it was 5+1. The losses are better on IP as I am very sensitive to carbs. You can exercise on both, though with IP you need to readjust the number of meals you take to be on the safe side.