good point Boris
Solid luggage is an advantage, but that said, we've never had any problems with soft luggage with locks on. Semi-rigid has he advantage of being lighter, which can't be discounted when you have to lug them through a hotel. JayThe lugging to and from the digs/hotel is something I've learnt over the years of touring, I try and make sure my luggage is comfortable/light enough to do in one trip. Don't forgot you'll have a helmet to carry and possibly a tank bag too.
I use bags inside the panniers and leave the boxes on the bike