I only used the Couch to 5k so far but it's the free version. The only thing I could see different with the purchased one is the ads are gone. Once I hit start and turn my music on, I don't touch it again so the ads don't bother me. Maybe you can try the free version and see if it works good enough for you?
Good luck on starting, I'm doing Week 3 Day 1 on Monday. I'm loving it so far, I hope it continues!