The 20PFI has a big toroidal transformer inside to help with blocking A/C noise. It also has the
Power Factor Technology "current reservoir". This is a big capacitor inside the power conditioner which gives faster immediate current to your devices which the A/C line would not normally provide. The 20PFI is very expensive, but these two items really provide a lot more.
The 15PFI also has the "current reservoir" capacitor, but lacks the toroidal transformer. It's less expensive, but still better than the "DMI".
The 15PFI also has the "current reservoir" capacitor, but lacks the toroidal transformer. It's less expensive, but still better than the "DMI".