I would say I need two backs, one for color and one for B/W. Depending on the scene or where I'm at I'll choose one of the two. Plus if I know I'll be in and out of building all day like in London it would be good to have Portra 160 in one back and Portra 800 in the other.
I have the 80/2.8 right now and thinking of the 55/2.8. My bag is small and I only have room for the camera, a 2nd back and a 2nd lens, 4 filters and 8 rolls of film. Looking back at all my 35mm travel shots I'd say 50% are taken with my 50mm, and 30% with my 28mm, and even those seem a little too wide, so the Mamiya 55mm should be perfect.
Eventually I'll get a bigger bag and add the 150/4, but I'm not dumping a lot of money into this system since its only a travel/vacation kit, and all of my "Serious" work is done with my RZ.
Thanks to everyone who helped make my decision easier!