I think Ceridian's product is better than ADP's and they are certainly lower cost. Ceridian includes significant HRIS functionality that ADP does not.
That being said, don't focus only on the amount of money saved. Make sure you also look at how many hours are freed up to do other productive work. People value freed hours in many different ways, sometimes at a multiple since the freed hours also count as a reduction in the rate at which headcount will increase if business increases. If any of the people who had their hand in payroll (e.g., accounting) also did some other productive work that you can put a figure on (e.g., accounts receivable), you can stack that on there, too. More time spent collecting money is more fun in the boss' eye than more time spent spending it.