Payroll deductions and automatic withdrawals are both methods of automatic saving. In both cases, you determine the amount you want to contribute and you can adjust that amount at any time.
Payroll deductions let you make contributions to your RRSP+ at each pay and enjoy your tax savings immediately right on your paycheck.
Automatic withdrawals let you contribute to your RRSP+ at the frequency of your choosing, but without the tax savings credited to you at source. You still benefit from tax savings when you file your taxes each year, just not on each paycheck.