Account Statements give you a way to generalize / summarize an Account's financial details; for one or multiple matters; and list details specific to the Account, it's associated Matters, Billings and Payments - for the time period specified upon creation.
Invoices
Invoices give you a way to generalize / summarize a Matter's financial details and list details specific to that Matter, and it's related Billings and Payments - for the time period specified upon creation.
You can choose to either formulate the appearance of the account specific Account Statement the same of different from the way you configure your matter specific Invoice.