Even though they both look the same, and the frames are interchangeable, there is a huge difference in overall performance and control from the EXP to the B-Series. As far as flight, there is not that much of a difference, the controls are exactly the same. The biggest difference comes when you start to hover and do precision controls. The B-Series loads up much nicer and holds its position better. It also gives better feed-back during flight.
For someone who hasn't flown much, the difference is very minimal. For someone who has flown a lot and really knows how to fine tune the kite the difference is quite a bit and well worth the extra cash.
I would say there is less difference between the B-Series and the B-Series PRO (which is somewhat less) than there is between the B-Series and the EXP (which is quite a lot).