The size aspect
I've been directed towards the upcoming Xperia Z3 Compact, due for UK release this month. I'll want to get hands on with a few devices before I decide though.
If you're already on a contract and just moving phone, you can't really go wrong with the Z3 compact IMO.
However, if buying PAYG then the Xperia Z1 compact would give you better value, without losing anything more than 4K recording and extended PS4 compatibility:
http://www.sonymobile.com/gb/products/phones/xperia-z3-compact/
I am still so surprised that Apple hasn't tried to release a waterproof iPhone.