There will be very little difference in the sale price at this age between a car with full dealer history and one that has partial dealer history and partial specialist. So you have nothing to lose long-term by opting for service at a specialist.
In terms of cost, the next service will be just a Service A with no additional items, so a cheap one, and a specialist will be cheaper than a dealer.
That said, the following service, in year 8, will be a big one and comprise of Service B, brake fluid change, air filter change, spark plug change (if petrol), and fuel filter change (if Diesel).
If done under a Mercedes Benz Service Care Plan, this service will work out cheaper at a dealer than a specialist.
The service after that, year 9, will again be Service A with no additional items, so cheaper at a specialist.
The following service, year 10, will be a big one again, and will include Service B, brake fluid change, and ATF and filter change (if automatic), so again cheaper at the dealer under the MB plan.
The bottom line is that if you opt for an MB Service Care Plan that covers the next 2, 3, or 4 services, then the overall cost of a dealer service will be roughly the same as that of a specialist.
(The above assumes that you car does average annual mileage and thus get serviced once a year, obviously if the annual mileage is high then it will get services more frequently)
The link below will show you what is due on the next service:
The link below will show the cost of an MB Service Care Plan:
servicecare.mercedes-benz.co.uk
See also:
I was just about to put my E500 back on a service care plan as I paid cash for the cheap A service last year. The next two are going to be the pricey ones - spark plugs, gearbox oil etc ... The plan was £42/month for two years but Mercedes are currently offering a 20% discount if you quote code...
forums.mbclub.co.uk
EDIT: and, as Will said above, the dealer service will also provide you with Mobilo roadside and recovery assistance for one year from the date of last dealer service, covering the UK and Europe.