From my reading of what @StevenS gave the link for there appears to be no difference between the Serif or AppStore purchases… both give the 30 day trial and then you can upgrade either for the app or to the Universal License version… with purchases from both stores you have to activate the license by logging into your Affinity account or making one and once done your license is valid for either the single app or the universal license depending upon which one you purchased.
So it seems that the only real difference is how you would like to update the app/apps… automatically via the AppStore or manually unless it is now automatically done from Affinity/Serif.
Affinity Publisher will most definitely be getting my money when it hits the streets!!!!
With such an incredibly responsive development team Affinity completely ***the proverbial*** on Adobe!
way to go folks... this trio of apps is awesome!