Go with the Mamiya 645 because it offer quicker respond, and TTL will greatly help with fill flash & AE kick ass. I own a Mamiya C220 & C330 with 4 lenses, which is nice enough for my need, but would love to get the Mamiya 6 or 7 with wide, medium & tele lenses (as a dream field camera).
Most job would be fine with the 35mm because it offer the quick respond that couldn't be match by a medium format, unless you are familiar with the situation. The type of photo that you shoot can be enhance by the type of camera you get such as field or studio.
I prefer the 6x6 format because it offer you the ease of formating so you could concerntrate on your subject, then crop in the stutio for the type of shot that you want...But most modern stock photo is gear toward magazine layout, therefor the portrait format is the type of shot you are most likely look for, so a 6x7 or 6x4.5 will do the job just as well as the 6x6 format.
But the most important of all on chosing a camera is the person that have to handle it. You have to get both camera out for a day and fire a few rolls of films (in the studio & field) through it befor you could make a good educated decision on what camera to get...to fit you personality, shooting style, and hand.
I have not use any Bronica, but have owned & uses Contax, Fuji, Hasselblads, Mamiyas, Pentaxes & Rollie medium format, and a few large format both 4x5s & 8x10s.