D40 or Metz
I use both a D40 (yes made by Sunpak per the made in Japan stamp on it) and a Metz CL-4.
I mainly use the D40 because of the LIGHT - the quality is superb being a "bare bulb" type, so its a different unit to the Metz thyristor head, which sufferes from the usual flash issues. The circular reflector gives perfectly even coverage over the square frame.
The D40 as others said sits superbly on top of a prism finder avove the lens axis. It is limited to TTL mose as it has no manual or aperture modes. use the 503 flash/film speed dial to control exposure compensation.
I occasionally use the Metz which has good coverage of the 2quare fram if you use the wide angle diffuser or a StoFen diffuser (the latter is best).
BUT, both suffer from limited guide number / power. The ideal unit might be one of those neat Quantum units with bare bulb and rechargable Turbo batteries - big bucks; but delivers superb light and controls.
So, the optimal set up is a D40 (for convenience) plus (for more power) a Quantum.