I'd go with the pre-paid assuming you get the price right, last few MBs I've gotten it for way lower than the cost at that time from their service dept, this is an item the dealership has a lot of latitude over even if they tell you service is a separate dept, it's all the same dealership. Of course don't forget, you need to keep the car long enough and then there's the loss of use of the money during that time and last, if you roll it into your loan or lease it'll cost you interest so have them sharpen their deal.