I've just recently changed from a Nexus 4 to a Sony Xperia Z3 Compact.
In terms of usability, I find them mostly the same. You're not going to notice performance differences on top end phones, apps are the same regardless of hardware (unless you like the manufacturer bloatware) however the battery life on the Sony is amazing. I can easily go two days without a charge.
I like the dedicated camera button on the Sony too. Makes it easy to launch the camera from the lock screen when you want to take a quick photo.
Just got this phone today. Have you got a case for it (which you could recommend?)