I haven't seen any of these in person. Going by specs, the Metz has the advantage of a bounce head, and the Nikon has five manual power levels, so if one of those features is significant for you, it might be the decisive factor.
But... both of them seem bulky enough to be a bit out of character for an R-D 1 outfit. I don't know how much luck you'd have finding one, but the flash unit I like on my R-D 1 is a Canon Speedlite 011A:
This is a shirt-pocket-size unit, with a thin, upright form factor that leaves both the finder-frame selector and the shutter speed dial unobstructed. (It looks as if that might be an issue with some of the units you mention.)
The 011A runs on two AA batteries, and has a very simple thyristor-controlled auto system: on an R-D 1, you set the ISO to 400 and the aperture to f/4, and that's it. Not much for control, but you can tweak apertures and exposure compensation to vary ambient/flash balance. It dates from Canon's A-1/AE-1 era, so may be scarce now -- but if you find a cheap one in a camera-show "junk box," it should be well worth picking up.