Adam, I think I know the cliff cavity you're referring to at Balconies - a prairie falcon pair has often used that site in past years for nesting (including last year). I was out monitoring South Balconies a week or two ago and a prairie falcon pair is definitely in the area again.
As for the difference between prairie and peregrine falcons: they are pretty much identical in terms of tail shape and size. The main differences are in coloration and plumage. Prairie falcons are brownish on the backs and crowns, and have whitish chests / undersides with some dark spotting on the chests that is not usually visible in flight. They have facial striping that looks a bit like a mustache. Viewed from below in flight, they look whitish, but with distinctive dark "armpits" or auxillaries.
Peregrine falcons look steely gray on the backs, and have distinctive blackish hoods on their heads. Viewed from below in flight, they look grayish due to consistent banding across body and wings, with a whitish patch of throat feathers sometimes visible as well.
Both species are cliff-cavity nesters and highly territorial. They both have very similar wailing and "kekking" alarm calls. At Pinnacles, the only confirmed peregrine falcon pair is at the Tuff Dome / Frothy Flake area, though there may be a peregrine pair moving in near Yaks as well. Otherwise, nesting falcons at the park - at High Peaks, Chaos Crag, Resurrection Wall, Pig Canyon, etc. - are prairie falcons.
Hope those details aren't too long-winded. Thanks for the observations!
