Accounts vs. keys
An account-based vault can reset your access if you forget your PIN. That's convenient, and it also means the service has some route back into your vault. A zero-knowledge design like PrivateRoll can't do that. If you forget your pattern, nobody can help, but nobody else can get in either.
Which trade-off is right for you?
Neither approach is wrong. If losing access is your biggest fear, an account with email recovery and a cloud copy may suit you. If someone getting access is your biggest fear, a key only you hold is the stronger choice.
Switching
Export your photos from your current vault to the Files app, import them into PrivateRoll, and then delete the exported copies. Our guide on moving between vault apps walks through each step without leaving copies behind.