Basically, the only thing that the two sets have in common is that they both have planes. However there is quite a bit of difference on the inside of the planes. There are a lot more details in the Friends plane. In addition to the seats, there is a rolling service cart complete with refreshments, emergency lighting as well as a larger galley and lav in the back of the plane.
Unlike the City set, there is no control tower or truck that services the plane. Instead, there is a passenger check-in/baggage carousel, a rolling staircase to gain access to the plane, plus the airport entrance that includes a gift shop and a café (or, one could argue, something that is very much like many other Friends sets out there). There are only three mini-dolls in this set compared to the six in the City set.
Since around 2000, the Lego Group has been promoting "Lego Serious Play", a form of business consultancy fostering creative thinking, in which team members build metaphors of their organizational identities and experiences using Lego bricks. Participants work through imaginary scenarios using visual three-dimensional Lego constructions, imaginatively exploring possibilities in a serious form of play.