We are heading to Corsica and the Dolomites can you please recommend what type of harness I should get – a sit harness or a full body harness as I have had conflicting advice ?

Usually this is just a matter of personal choice unless your tour guide insists on a particular kind of harness:

A sit harness is fine for general uses and so will serve you for other needs, combined with an inexpensive  chest harness it gives almost the same benefits as a full body harness.

Full body harnesses is the convention solution though. They are generally more comfy for extended wear and are better with top-heavy loads (such as a backpack) as it is possible to attach the rope/via ferrata device to a higher point above your center of gravity which will help prevent inversion if you should fall. 

Its worth bearing in mind that for industrial applications a full body harness is always used which is testament to  advantages when used intensely all day long and to give maximum protection, freedom of movement and ease of fitting.