My advice is definitely the Nikon. You open yourself up to the world of Nikon AI and AI'd lenses, some of which are rare, specialized, and very, very functional. Canon uses the newer mount that 's only for their new AF lenses, whereas Nikon hasn't changed their SLR mount in, like, ever. If you want to do stopdown metering, you can use ANY* Nikon SLR lens every made, EVER. I've used Nikons and Canons, and let me say I love my N2000 and my AE1-Program both very much, but the N2000 is a better camera. I know the AE1-P is old, but I've also used an EOS-Rebel-G extensively and I find the N2000 to be roundly superior, even though it lacks automatic focus and powered rewind. (not that the latter matters a good goddamn on a digital!!!) You really, really want the ability to use good manual focus lenses, as well, as they have focus scales and such that are helpful for the dark, for exacting work, and for flashguns especially. There are also times, when for reasons creative or practical, you want the ability to override the automatic aperture controls, and the old Nikon lenses let you do just that. Remember that you can still of course hitch on a new AF-Nikkor and go point-and-shoot; but the ability to "old-school" it cannot be underrated. GO NIKON!
* CAVEAT: You can't use the Nikkor-IX lenses off the Pronea APS SLRs (but those sucked, like all APS crap,) nor can you mount Nikonos underwater lenses, but why would you want to?
. Those SLR's are like a thousand dollars. But from what I've heard from my supervisor, the Nikon's tend to be better for digital SLR's because the Canon ones add the other lenses on top of the existing lens, potentially damaging the shot whereas the Nikon's are removed and new one's placed on.