I have both Pentax and Fuji.
The Fuji GA645zi is the lightest, but of course the zoom lens is fixed on the camera. Think "point & shoot on massive steroids." It weighs 2 pounds, and is quite compact. A 52mm filter fits on the lens. Aperture and shutter speed can be manually set, but it's manually zone focused. It would have been utterly awsome if it was a real rangefinder too, though.
The Pentax weighs four pounds with 75mm lens, and is slightly bulkier. The interchangeable lens makes it a much more versatile camera. I feel much more comfortable changing the film, as the Fuji has the rear element exposed, while the shutter is between you and the lens with the Pentax.
If weight is the most important factor, use a Holga. At 8oz, nothing beats it yet. For quality of the image, I can't find fault with either the Pentax or the Fuji. The lenses on both are superb.