CSV: maximum compatibility
CSV is plain text — a comma-separated table that opens in Excel, Google Sheets, and virtually every accounting tool. It's the right choice when you're importing into other software (QuickBooks, Xero, Wave, a database), because almost everything reads it.
The trade-offs: no formatting, no formulas, and a single sheet. Leading zeros and long numbers can also be mangled if a tool guesses the column type — so for things like account numbers, Excel is safer.
Excel (.xlsx): formatting and formulas
Excel keeps number and date formatting, supports formulas, multiple sheets, and column types. Choose it when a person will work in the file — categorizing transactions, building totals, reconciling — rather than feeding it to another system.
Quick rule of thumb
Importing into other software → CSV. Working with the numbers yourself → Excel. When unsure, export both — they take seconds to produce from the same statement.