The "Twice a month" option splits your monthly payment in half and is debited on two different days you choose per month. This option may help you in budgeting your payments.
The "Every two weeks" option allows you to make payments every other week on a day that you pick. Depending on how the calendar falls and the day of the week you choose, about twice per year you'll be making three half payments in a month. These extra half payments will get applied to principal and reduce your balance more quickly than if you were to pay monthly or twice a month. This option helps you pay your loan down faster AND may help you with budgeting.
Overall - Your monthly payment is applied as of the payment due date, so no matter when you make your payment the timing of the payment won't change the amount of interest paid. This is because all of the month’s interest being paid was from the previous month. If you decide to include additional principal, it'll impact the amount of interest you pay over the life of the loan and will change the amount of interest paid in the next billing cycle.