I used the GW690III quite a bit. Some thoughts to consider:
-It's not really all that portable. It feels like a giant 35mm RF and is quite cumbersome to handle at times.
-The shutter on B/T setting does not close after 1 second, rather you must place the lens cap in front and then click the shutter shut.
-Extremely sharp at most apertures except at f/32 I noticed it began to look quite poor.
-It is RUGGED. I dropped it down the Grand Canyon about 400 feet with no scratches, no dents, only a broken filter.
-If you're going to buy a GW670III, why not get a GW690III and get the extra negative space. It's the same camera, just with different masking and counter. Same size, bigger neg.
-Lastly, consider the GF670. I finally got to handle one and it's fantastic. Small, fits in a breast pocket, meters well, and can be used on a 35mm tripod without complaint.
Just some thoughts to consider. They are great cameras nevertheless, but I figure I would share.
EDIT: I used the Mamiya Universal at the same time (funny how you ask) and can say they are similar in size and weight. Granted, the Fuji GW670/690III is lighter, but they really are quite similar in size. Seriously consider the GF670.