I didn't notice that the sound of the motors in your first video was particularly bad. Is noise or vibration a particular problem? Steppers tend to be like that.
It's difficult to recommend motors, because we don't really have any detail about your application. If you want to drive the motors in unison, then I think it would be better with a single controller and using Step/dir. A Mega has plenty of pins anyway, it might be easier to sync those than SPI.
But I guess if you are spending $100's on controllers and drivers (probably unnecessary), you don't want to skimp on the motors. The motors may be the most critical part.
It's difficult to recommend motors, because we don't really have any detail about your application. If you want to drive the motors in unison, then I think it would be better with a single controller and using Step/dir. A Mega has plenty of pins anyway, it might be easier to sync those than SPI.
But I guess if you are spending $100's on controllers and drivers (probably unnecessary), you don't want to skimp on the motors. The motors may be the most critical part.