I'd use the single field, noting difference between deposits and withdrawals by the sing, as it seems more intuitive. At any point, the correct balance is fond with a simple aggregate. The only reason I would see for the two field approach would be an aid to the handicapped who might not rember to enter / use the sign in their calcs.
Of course the entire (entry) process is specifically for the user's benefit, so I would consult them or even possible offer a variety of forms and let them choose. After all, the real issue is to maintain the balance according to the data, ans possibly shopw various segments / perspectives.
In almost any useful (cheek register) application, the user would also want to be able to identify categories of both income and expenses and total them over periods, e.g. where did it come from & where did it go.
MichaelRed