Well the RPi's of course are the safe ground for any SBC user, with enough support and software to make them far and away the easiest systems to use.
I took delivery of an RPi Model 3 the day after launch, knowing that they'd probably sellout.
It of course presented no problems, once I had downloaded the latest versions of the Rasbian distro which could properly set up the 64bit system. The older versions being unable to do so.
The biggest new thing on the Model 3 for me, was the wifi, which works brilliantly. I was unable to figure out out to get Bluetooth to work but since I don't really plan to use the machine as anything other than a target for game dev I didn't really care. I am sure someone will find a reason to use it on the forums.
The speed of the Model 3 is not exactly blisteringly better than the Model 2 but it is there, the GUI in particular feels just a bit smoother and more responsive.
It seems to make little to no impact on my projects, which still run at reasonable 35FPS with no optimizations under extreme stress of rendering 3000 software sprites.
I get pretty much the same performance on the Pi Zero, but then again I am only using 1 core so can't expect the Model 3 to be massively better.
As with all RPi's the Model 3 is a nice safe and secure upgrade that presents no technical issues.