The difference lies in data.
If you disconnect
All data sync stops.
That is, your Federal Bank Savings Account on Fi & your other bank accounts will stop communicating.
If you delete
Fi cannot retrieve any data.
Plus, all historical data will also vanish.
It also affects your search results on Ask Fi.
When an account is disconnected:
Data that was previously provided to Fi stays on the app. So, insights & search features will work on the existing data.
When an account is deleted:
-All data is lost, like it never existed.
In addition, Fi will no longer be able to communicate with your bank. Your Ask Fi search results will get affected as well.
Bottom line
Disconnect: Previously retrieved data is seen & Ask.Fi can be used on that data.
Delete: Absolutely nothing will be available.