You can’t have two QuickFile accounts with the same email address, you would be better to set up the Affinity account first and create the account through Affinity.
Affinity means that you only have to have one log in to access the accounts - so you log in to Affinity and then from there you can chose which QuickFile account to enter.
The power user subscription gives you additional features which you won’t get through Affinity. There are some guides which may help:
Power User Subscription vs Affinity
QuickFile Affinity overview