I use the Bronica and have had absolutely great results with it. Great optics and quick/confident focusing. I also use 6X7 in a Pentax and I'm certain I get a much higher percentage of solid shots when using the little Bronica. My normal printing is 8X10 with 11X14 at times and the Bronica is more than up to that task.
I've almost popped for the Mamiya a couple of times and each time, I set it down after handling it and decide against it. It's likely not the best reasoning and I'm certain it would give great results but the plasticky, hollow feeling of it is not very gratifying and making out a large check for it and its pricey lenses just seemed too difficult. You've got a nice decision to make as I'm certain you would be very pleased with either one as a solid tool.