Like others have said, I think the two cars appeal to different buyers/markets. Trading from a G37 coupe to an M3 makes sense, as does trading from a 335i to an M3. Similar sizes, similar smallish/sporty function. The 6-series is in a different market size-wise and $-wise. More luxury GT, less edge. I think of the 650i and M6 buyer cross-shopping the cheaper Astons and Maserati coupes.
I actually traded from an E60 M5 to my E90 M3. Big step down $-wise, but big improvement in terms of handling/rawness/track-readiness while still being practical. If in a position to upgrade from the M3 and if looking for a similar package (fast, sporty, bit of an edge, great handling, European quality and flair), then the 911 does seem like the next logical step. Although the ideal move up would probably be an E9X M3 with the latest BMW electronics/interfaces, LED lights, factory BBK with optional aggressive pads, supercharger, and coilovers, all BMW-approved and covered by warranty.
