The TFC45-ⅡB is the non-folder right? I bet the number of users is fairly small. The folder is much more common.
I've got a 120mm for 6x12 but like David said it's a personal choice.
I've got the 612BC roll film back. The downside of my model is the very dark ruby window. I just picked up the SPC617 back which fits the 5x7[and bigger] cameras. It's got a more reasonable ruby window. I don't know if that means newer 612BC have similar windows. The intention is to use it on my 5x7 or 8x10 with the reducing back.