I am another advocate of the D5. Much quieter than DDC, incredible reliability and longevity, and no heat issues like the DDC.
PWM vs Vario is completely a subjective preference. I have always preferred PWM as it gives speed control through software, and curves can be set up to accommodate the load as needed allowing for the lowest noise possible in every situation.
Between the XSPC and Alphacool, the pump it self will be essentially identical, but XSPC does make a nicer quality top if you are purchasing complete with the top. You can also buy a bare D5 and the top of your choice, or a res with D5 mount allowing for a combo.