gravity or booster or power shower

[surfcob]

Hi all,
For new affordable 2 storey 100m2 3 bedroom semi detached with air to water heat pump and system mixing valve, which should I go with:
Option 1: gravity domestic water with pumped shower (mira vigor)
Option 2: whole house / stuart turner monsoon - which model
Option 3: whole house / grundfoss scala 2
Best regards,
A

 

[Aire]

Is there a reason you can't have an unvented cylinder? Cheaper to run and less maintenance.