The Play has a amp, the Ultra does not, hence the price difference. Do you need an amp?
Ease of use is critical for me, so I would look at the UI on both the phone app and on the screen. I have never used a Play, but my impression is that the UI on the Play is more flexible and customizable than on the Ultra, but you should check that out.
Some other things to be aware of.
The Play plays DSD without converting to PCM like the Ultra does. It also supports higher PCM rates if that is of interest.
The USB input on the Play supports both storage devices and a PC. The Ultra only supports storage devices.
The Play has a feature to mirror the screen to your phone.