On a high level, it doesn't matter. Both provedes even and close racing and that's the main thing.
The beauty of the HP limit is in its simplicity. 532HP and that's it. Just go racing, no discussions.
A PP based system offers more opportunities but is more complex. You can Aero/HP optimize the car between different tracks so you get more tuning possibilies. Also the PP can be adjusted in steps of 1PP where HP only in steps of 10HP. This gives better possibilities to rate different cars and create an even level playing field including more cars including personal PP ratings for fastest/slowest drivers.
The main "invention" the Euro series introduced is the, enter the track starting with the lowest PP and then adjust upwards rather than all at once. I had my doubts that people would have the dicipline to make it work, but they did, and it worked beautifully.
Given above, I think a PP based racing system is actually better than HP based, but it has to be withing a racing group knowing each other very well.
It's been a while since I last raced GT but and decided to create a lobby tonight for some casual GT500 racing if anyone is up for it. Hope you join the fun!
Room Number: 1472 - 6399 - 8665 - 9407 -.9478
Race Laps 10 (Soft tyres, probably 1 pitstop)
Closed.
