I don't see too much wrong with a solid coupling if the alignment is perfect. In a face mount situation, this is usually not too hard to accomplish, if the mounting surface has been machined perpendicular to the screw. In such a case, then the motor can be mounted on the screw, supported only by the coupling, and then the motor screws can be tightened.
If you cannot guarantee the perpendicularity of the face, then use the helical coupling.
For driving an encoder, I'd always use the helical coupling because of the possiblity of damaging the delicate instrument bearings in it.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)