The correct answer is that if the service schedule dictates both annual and mileage services, then that's what should be done. The manufacturer obviously deems it mechanically necessary, but the manufacturer ain't paying for it!
My rules of thumb would be...
If the car is nowhere close to triggering the mileage service within an annual cycle, then the annual service stamp is most important followed by the mileage service stamp. This would likely be an expensive purchase. A garage queen / collection car.
If the car is very much over the trigger point for a mileage service within an annual cycle, then the annual service stamp and mileage stamps are equally important. This could well be a bargain if supported by associated receipts, as it shows enthusiastic and careful ownership. The engine on the 4200 has proven it can take the miles, in spite of purchasers being scared off by high mileage examples.
If the car is annually close to triggering the mileage service, then the mileage service stamps are the more important, and I wouldn't be worried by the lack of annual service stamps. I'd expect average pricing to apply based on spec and mileage.
If the car is annually close to triggering the mileage service and has both mileage and annual stamps, then this too could be a bargain and demonstrates careful ownership, but without being a garage queen. I suspect these would be rare as hens teeth though.
