I don't know much about Prairie Spy
I am planning to try Nova Spy this year(spring though), as a local orchard said they were growing them and liked them (I think selling as Northern Spy also). I understand that they might not get quite as large as some specimens of Northern Spy apples do.
At age 58 I don't have time to wait for the Northern Spies to come into bearing :)
I also would like to hear from anyone with personal experience.

A friend's Prairie Spy produced wonderful-tasting apples of decent size here in W. Montana. I have a graft on my tree but it hasn't put on much growth yet and I don't expect it to fruit for two-three years at least.

It is possible that I am already buying Nova or Prairie, since those I buy are medium sized. I note that they do not bruise as easily as true Northern Spys. The eating quality is similar.

Spigold comes into bearing much more quickly. My Northern Spy is "building my patience". :)
John S
PDX OR