There's also a 33-55mm zoom for the Pentax 645.
I've had a Mamiya 645 Pro TL for the last few years, and I have enjoyed using it as a handheld SLR. With the metered prism and the motor winder it really is a pretty good handheld camera, on par with my Nikon as far as feel. It is however louder than the Nikon. The motor makes the most noise, but it's not bad at all if you use the hand crank. But the motor grip makes the camera much easier to use handheld. There are two winders for the Pro TL, I use the smaller one and it hardly adds any weight.
I have had plenty of good shots at 1/30th or slower, as long as I brace my body. For me no SLR is as good as a rangefinder/TLR with slow shutter speeds (I must twitch as the finder goes dark).
I listed it for sale a month or two ago. There are some pictures of it here, but none comparing the camera to the Pentax.
I have a pentax 645 and a couple of 35mm's. I can not consistantly shoot 1/30 handheld without bracing against something with either system. The 645 is bulkier but I find the ergonomics good, and from eveything Ive heard the 55mm for it is wicked sharp. As an aside I'm thinking about getting rid of my gsn and perkeo 1 w/ vaskar lens to finance a spotmatic. Reason simply ergonomics
I have both the Pentax 645 and Mamiya 645 1000s. In terms of ergonomics, the Pentax is better for me because I am used to left hand focusing. The Mamiya with its left hand grip requires right hand focusing, which is certainly okay, but it doesn't feel as natural. Unfortunately, the split image focusing screen on my Pentax is rather hard for me to see unless I really take my time and look very carefully. I have many out of focus pictures that are either due to this, or the camera has a problem. I honestly haven't used it enough to confirm what the problem is.
The Mamiya is certainly heavier and feels like it is built like a tank. I've recently taken the left hand grip off to shoot for a while again with no grip to see if it feels any more natural and quicker to use with left hand focusing.
One disadvantage of both cameras is the lack of removable backs. All too often I have a half used roll of film in the Mamiya and wish it had a different film in it for what I want to shoot. If you think this might be an issue for you, you should probably consider the 645 Pro.
I haven't done enough shooting at very slow shutter speeds to say which is best.
You can always buy one camera and sell it if you don't like it and buy the other one.
Yes, Bill, I've been thinking about a 2nd (or someday a third?) body.