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.