I have had the Pentax 67 for a couple of years, and just bought into a GS-1 system. I haven't had the GS-1 long so my experience is limited, but a few comments below:
- The Pentax has a much wider selection of lenses, and with the exception of certain 'desirable' or uncommon lenses (75/2.8, 100 macro, 105/2.4, and some of the longer lenses) the rest of the lenses are surprisingly cheap, like $100 cheap.
- the Pentax lenses are very good (in my experience), even the lenses that nobody says anything good about. I haven't had the Bronica long enough to form an opinion.
- The Seiko #0 shutter on the GS-1 limits the lens apertures so most are f4 or slower with the 100mm and ultra rare 80mm are f3.5. no f2.8 or f2.4 for the Bronica.
- If you like the WLF style of shooting*, the GS-1 can be more compact and lighter. But if you outfit it with the grip and prism finder, is looses most of that advantage.
- even with the WLF, the P67 feels bulkier and heavier.
- the metered finders on the GS-1 can give you aperture priority, which you can't get on the Pentax without spending on the very expensive 67II.
- the Bronica has a removable film back system, which, on my ETRSi, I have found is a real benefit when I'm traveling.
In the end, and this is very subjective opinion, the bulk and weight of my P67 was more than I really wanted to deal with. A little after I got it, I also got an ETRSi with a waist level finder*. I loved the compact size and the ergonomics of shooting with a WLF, so it became my primary camera. I found the 35mm SLR ergos of the P67 didn't work as well for me when the camera was 5x the size of a 35mm SLR. So, after briefly considering the Mamiya RZ, I dove into a GS-1.
Make no mistake, the GS-1 is closer in size to a P67 than to a ETRSi. But I love how it handles so far, but again, that because I love the handling of a "hasselbald" style camera (i.e. a cube with the lens on the front, a film back on the back, and WLF on top . But I'll keep the ETRSi around for more portable use. I haven't decided if I'll keep my P67 yet.
* As many will point out there is a severe limitation to shooting a rectangular format with a WLF, because portrait mode is borderline impossible (I've done a few on my ETRSi). This drives a lot of people understandably towards a RB or RZ 67, but I was narrowly focused on portability, and the rotating back of the RB/RZ makes it larger and heavier. The reality is even with a prism on my many other cameras, I still shoot 90-95% landscape, so I just decided to accept the limitation. You may not feel the same way. I do have a prism for both my ETRSi and GS-1, but I rarely use them.