I myself have been asking a very similar question. I have an m6 that i shoot with daily. When it started acting up (damn exposure lights wouldnt turn off) i switched to my r2a (my first foray into rangefinders). After a few rolls, i decided it wasnt for me. Mostly due to the irregular spacing between shots on the roll. Not so good if you scan your negs, as i do. I'd love AE, so i thought, hey, i'll look into used m7s.
At a local camera store yesterday i played with the ZI for almost an hour. Tried to get an overall feel for the camera. I also played with an m7. Long story short, im going for a ZI in black with the 35 biogon. its a beautiful camera, all metal, just not as heavy, since its missing that big brass plate at the top. I'm willing to forgive some of the diffrences in weight, percieved quality etc, becuase at the price popflash has them at, its a steal. In my opinion.
Is the ZI a better made camera than the r2a? No question. Is it not as reliable, well built, etc than my m6? Not 100 percent sure. Is it lighter? Yes. Are the Zeiss lenses fantastic? Did you ever see some 6x6 chromes from a hassy??? Will i get rid of the r2a? Probably not. After selling my mamiya 6 years ago, i learned my lesson.
Let us know what you decide.