I do not have an SLR nor am I committed to any brands, but I'm debating between a 35mm or a 645. I was initially looking into 35mm cameras but for not that much more I can get a 645 MF SLR that might meet all of my requirements. My intention is to use this camera handheld and with enough speed to be used indoors.

What matters to me:
-handheld use and ergonomics

What does not matter:
-size, weight, and noise to a reasonable extent
-any auto-features such as auto-focus, auto-exposure, auto film rewind
-mirror lockup

I'm thinking mostly of a 35mm Canon FD or a 645 Pentax or Mamiya; I'm aware of the key differences between the two formats and brands (and I'm open to more options) but I'm trying to weigh the pros/cons of the 35mm vs 645 slr first before deciding upon which particular brand and lens. I'm mostly interested in the 28mm and 35mm focal lengths (in 35mm equiv) and I might pick up a standard in the future but not interested in anything really wider or longer.

My Questions
-Is it reasonable to shoot at the 645 SLRs 1/30s handheld? I can shoot at 1/30s on my C220 and 35mm FL rangefinders without much effort, but they do not have swinging mirrors...
-How does the noise and sound compare with the Pentax and Mamiya systems? I've heard that Pentax's are smaller and quieter but the Mamiyas bulkier and louder. Any pictures of the two side-by-side? I couldn't find any size comparisons online.
-Is the Mamiya ergonomic without the grip. I find that it adds a significant amount of bulk so I'd like to use one without a grip when I want to slim down.