My friend with a 550xi and I would switch occasionally. I think I posted a thread on it somewhere. To me... the 650 handled better, sounded better (had sport exhaust after all), and the steering was sooo much better. But the 550 was more nimble? The throttle mapping was just so different - it felt FAST at all speeds. They could not be more different cars! On that note, I think you will really like the 650, just make sure to put it in sport when you need to snap around town
Yes, the seats in the GC are different than the coupe and... not knocking the 650 coupe, but just like a stick in a big car, a large two door car is not my style. I've never actually owned a car as beautiful as the GC.
To me, the M2 is engaging - it wraps around you (as you noted), you become one with the car - kind of like the previous gen 3-series. Yes, there are certainly some imperfections (electric steering has a small dead spot and then TURNS like crazy), dakota leather, etc. However, I remember saying to my dealer a week after I got my 650 that I thought I might have made a mistake - whereas, with the M2, I just texted him last night that I was really really starting to like this car - winter in Chicago and all!
edit: That doesn't mean the M2 is a better car (or that the 650 is a better car), it just means that they are very very different cars and today, I personally prefer the M2. But in three years, who knows? That new 550xi certainly looks yummy.