Julie,
You've seen my bronica sq-a. From what I've seen the Bronica Gs-1 cameras are slightly bigger. The downside of the gs1 system is the lack of a revolving back, so you would have to tilt the camera, which means you would want a prism finder and a grip. Both add weight and bulk. The only advantage I see going the bronica over the pentax is the ability to change backs.