Why A? I know I love shooting with a TLR. By no means have I outgrown the 124G, but as I understand it, it's an entry-level TLR in this day and age (nothing wrong with that) and is considered many to be built like a "toy." I am in no hurry to get rid of the 124G if I choose route A, but I can keep my eyes open for a good deal on a cord/flex.
Why B? As mentioned above, by no means have I outgrown or mastered the 124G. I'm sure it will serve me as long as I take care of it. m645 because I'm interested in seeing if I will enjoy shooting that aspect ratio as well.
Another thing I should mention is I will use either for street photography. My experience with the 124G is that the general public seems intrigued and less threatened by it (I'm sure this goes for any TLR). For anyone with experience, how do people respond to the m645 (or like cameras) being pointed at them? I ask this because I am by no means threatening or creepy, but my experience with DSLRs with street photography has been poor. Maybe it's my approach, maybe it's the camera. All I know is that I've had much more success with the 124G than my 5D2. Wondering if I'll be as successful with something like an m645.
@benjiboy - I understand the limits of using a WLF with 645 and I am okay with shooting in landscape only. I rarely shoot in portrait with any of my 35mm cameras and I don't foresee landscape only being a problem if I choose to go 645. As for your suggestion about a MF like the C330, I've considered the C33 and C330 - but I've read bother are cumbersome to carry, especially for street photography.