... There are a lot more Pentax 67 bodies out there, and I would say the repair prospects are better- it's a totally mechanical camera, with a mechanical lens/body connection, ...
...I replaced them both with a Mamiya 7 ...
I will point out that the Pentax 67 is completely battery dependent, just like the Bronica GS-1. And the Mamiya-7. But that has never stopped anyone. (edit: beat me to it by minutes! ; )
I have owned and shot with both, but not at the same time. I had a Pentax 67 with the waist level and both prism finders, 45, 55, 90LS, 105, 135, 165ls, 200 & 300. I currently have and shoot with the GS-1, with the waist level, AE prism, grip, 50, 65, 100, 150 and 200. I have backpacked both, and used both with studio lighting.
The main difference between the 2 are: leaf vs focal plan shutter, and interchangeable backs on the GS-1. If these are important to you, then that should make your decision.
Both are packable, especially with their waist level finders. The Pentax is slightly heavier, but not much. Similar size. But if weight is an issue, the Bronica is slightly lighter, and packs smaller.
Lenses from the same era are equivalently good, the Pentax 67 has been around a lot longer, and some of their earlier ones are not quite as good. But P67 lenses are uniformly faster due to the FP shutter, which may be important to you.
The GS-1 does not have an instant return mirror.
I can't tell you which one to get, that has to be your decision (the above plus price /availability). I picked the GS-1, but I'd be happy to use the P67 in it's place.