There's a couple of other differences that are not mentioned. The AE-1 Ps have a user replaceable focus screen, which is a really nice feature. The focus screens in the Ps are a LOT brighter than on the AE-1 cameras too. If you use a non Canon lens w/ an adapter on the P cameras, you use the f5.6 LED in the viewfinder just like you would w/ a match needle camera. f8 gives you one stop less exposure, f4 is one stop more, and f5.6 would be the equivalent of having your needle centered. You cannot do this w/ a Canon FD lens to my knowledge, only w/ non FD lenses. THE AE1-P cameras also have an AE lock feature (located in a very unergonomic spot on the camera).
I like shooting non AI Nikon lenses on the AE-1P cameras just for the looks you get when you stumble onto someone who knows what's up. Using a Leica R lens on the Canon cameras is easy too w/ just a Leica R to Nikon adapter on your Nikon lens, then you put a Nikon to Canon FD adapter on that. As good as FD glass is, the image quality goes way up.
Unfortunately, many of the AE-1 and AE-1P cameras suffer from shutter screech, and/or shutter speeds that are slow, or run on only one speed. These cameras have lived far beyond their expected life span, which is why I'm trying to figure out if I can hack a Nikon mount to my FL 135 3.5 lens and use it on my Nikon cameras. Love the FD glass, the FD cameras, not so much. I prefer the Nikon SLRs to nearly any SLR for build quality, especially the early Nikkormats.