Raid, you are not on assignment, so the goal should be to enjoy the experience. Choose the set you feel more comfortable with. Every photographer has its personal tradeoff regarding weight, capabilities, travel plan and photo opportunities covered.
Photographically speaking: in my limited experience of Italy, a wide angle is essential. I'd guess that at least for the old city of Siena which would probably be one of your main subjects.
The SWC is close to ideal IMHO, regarding capabilities, if you can cope with the weight and size. The M9 with 21 or 28mm would also be an excellent choice. I do not really see the point of taking the M8 except as backup of the M9.
If you need 120 film it is much cheaper in the U.S.; consider taking along some ISO 400 film, for the interiors; grain is not that critical in 6x6 size, and HP5+ or TriX can be pushed to 800 without much degradation.
I assume your SWC can handle additional film backs?
Personally I'd go with a single camera, either the Hassy or M9, because I'd hate switching eye and mind from one type to another and taking care of too much gear. If I lose the main camera, my B-plan is usually to get a cheap P&S, or borrow my wife's.