A proper step up. There are few tougher fell races in the calendar (according to people who know about these sorts of things - Jura and the like)
Peris is proper mountains, skyline is part trail race with a good long bog trot to finish once the legs are sapped. At Peris the angles are all greater, the ups are more up the downs are more down, and the ground is mostly a lot rougher/rockier/grassier.
At 4500ft for Edale, a half Peris would be more similar - run on the same day and about half an hour earlier depart. So for a full peris think a similar distance to Edale only twice the climbing on rougher ground and you'll be on the right track.
Agree with Eddie, about an hour difference in time taken, given no route faff.
Are Both A Longs? - Chalk and cheese.