From purely financial point of view:
You can get the GF670 used for a very reasonable money today. Should you not like it, than you can sell it with little loss. On the other hand - the FX-N can only be had new and is costly (around $6k I recall) and should you not feel content with it, you will sell it with at least $1k loss.
I know what would I choose (FX-N if sky was the limit, the rendering of the GF670 lens does not appeal to me), but it is your choice.
I would definitely try to see many photos taken with both cameras to get the impression what kind of rendering one could expect.
You did not hear much about the Rolleiflex here because only handful of users have the FX or FX-N.