Current reserve (peak current) is 55A vs. 45A. Your amp takes current from the mains in form of current spikes that have much higher amplitude than average current. It is difficult to say if you need 55A or 45A reserve. I got 20PFI used (almost new) for about $600 to be ready for any amp I can get in the future (and also because it was very good deal) - otherwise I would get 15PFI (I don't listen very loud). AFAIK it does not have any perishable parts, so I bought it used and intend to keep it forever.
Power factor correction is most likely combination of large inductor and large capacitor that stores energy and can deliver up to 45/55A peak. It would be hard to estimate current crest factor (narrowness and amplitude of the spikes) because it depends on design of an amp. So, if you don't listen loud get 15A version otherwise 20A. With available current it is always - "the more the better".
Power factor correction is most likely combination of large inductor and large capacitor that stores energy and can deliver up to 45/55A peak. It would be hard to estimate current crest factor (narrowness and amplitude of the spikes) because it depends on design of an amp. So, if you don't listen loud get 15A version otherwise 20A. With available current it is always - "the more the better".