Know (to me) differences:
1: As stated P heads' spark plugs go in at a different angle. This puts the spark at a better place of the combustion chamber, but P heads don't work with any header. Non-P heads do.
2: P heads are known to have a slightly smaller combustion chamber, 58-61cc vs. 60-63cc. This will make for a slightly higher compression ratio.
3: P heads have a slightly smaller exhaust valve, 1.46" vs. 1.54". Both heads have 1.84" intake valves.
Need4spd90 uses P heads and I use F3ZE-AA non-P heads. I believe our cars produce very similar dyno numbers, so take your pick. Both are notably better than stock E7TE-PA heads.
'94 Laser red 'vert. Still a 5-oh... for now