What should I take for a trip to South Italy?

If you love taking photographs, it's worth the risk. But the risk isn't that big.

As for what to take: I would go with the 9-18 as a main lens, 25 and 45 for portraits, and maybe the 12 for interiors. leave the big one at home. As for which camera, or both: I think one will do, you seem to shoot mainly static subjects. Landscapes and buildings will wait for you to change lenses. On the other hand, having a backup is not that bad. It doesn't mean you have to carry it with you all the time.

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Regards,
Gravi