The HW75 has a better trigger and build quality but it is only around 2ft/lbs in power. This is fine for 10m paper but you may find it lacking on longer nockover targets.
The HW75 is all metal construction like the HW45. Single stroke operation only.
The Alecto trigger is not quite as good but still nice to use. External construction is plastic, but it is well made and durable. Multi pump give you more power (up to 5.5 ft/lbs) than the HW75 but you also get more muzzle flip and movement at the higher powers. It is still an excellent pistol for the money though.
For nockovers probably the Alecto, for pure paper punching probably the HW75. But you need to try both to see for yourself really.
HTH
Mark