I would not worry about output level of the headphone amp, although ‘loud enough’ is not very clearly defined and is also very dependent on your headphone impedance.
Personally I would never buy the Novation for studio work. It has no useful inputs other than a single stereo RCA which is completely useless if you ever want to record a microphone or guitar for instance.
I have no experience with Ableton, the only DJing I ever did was with Traktor. With traktor however, cueing was very easy in the software and also on the hardware controller I had. I never felt the need for a button on my audio interface for that purpose.
It’s your decision of course, but the UR44 is simply much better for studiowork and although the Novation may have a slight functional edge for DJing, the UR44 is also fine for that.
If you are still in doubt, you can try to find a local dealer that will let you testdrive the UR44 with ableton, just to see if that floats your boat.