Is one option better or worse than the other? They both have the same outcome, right?
If you are good at saving, then you should theoretically be better off earning interest on your £100/month payment, ie, taking the second payback plan. However, if this describes you, then you won't have needed to borrow the money in the first place! People generally are quite bad at budgeting and scheduling for large payments, so it is overwhelmingly better for 'Joe Public' to take the pay-per-month repayment plan.