The Yashicamat is pretty portable. You are getting advice over on apug to not sell the Mamiya and I would concur with that - unless - you are sure you're never going to use it again. Bottom line: your decision, not ours.
I would second the Fuji for "portability". I have the 645S. The lens is a 60mm, about equal to a 35mm on 35mm! Sharpest lens I own if you are into that sort of thing. I also have a Mamiya 645, Mamiya twin lens, and an RB67. Use them all, but the Fuji is for travel. The twin lens is actually for "carrying", since it is a lot lighter than either of the SLRs. I can put the body and three lenses in a small backback and it weighs much less than the SLRs.