Having spent ALOT of time (Summer season in Alps, a trip to BC/Alberta) on an original Mk1 520 PA and then swapping to a Ragley BluePig (Mk1), I’m probably partially qualified to answer this.
I would suggest that you won’t feel that much difference between an old style 456 and PA.
However if you try a 456 Evo or BluePig (X) you will notice a big difference.
The PA with a 130 or 140mm fork, tended to tuck under when things started to get hairy. The BP doesn’t do this as the HA is soooo slack.
I think your choice depends on what sort of riding and how a long a fork you plan on putting on the bike.
Up to 130mm, and slightly flatter terrain. Go PA.
Over 130mm, and steeper terrain. Go BP.
Also a factor for me was the fact that having had a 17.5in PA, they changed the sizing to 17.0in, which meant even more seatpost out of the frame or going up to a 19in.
I currently ride an 18in Bluepig.