Hi , as others have suggested the difference between an MB dealer and a specialist service is not as large as people suggest.
I held fire on commenting about pricing this time around.
First off - the basic servicing prices are high. MB have used the service plans to set an expectation. In particular prices for ATF and oil fluids and spark plugs tend to be a rip off.
Second off - maybe it's just our dealers - you go to a main dealer you speak to a customer agent - it's not likely the person at the desk is somebody who works on the cars. I have liked the indys that basically let you speak directly with the mechanic.
I have an old Touareg. It's not a simple car. So far the annual costs I have had over the last two years for servicing and unscheduled work (front disks + pads, front bearing) have been about the same in total as the two year service only costs quoted here.
The equivalent of Mobilo is worth about £53/year to me. We have household Green Flag personal cover. Back when I had a MB the advantage of using an independnt recovery outfit was that they would recover the car to a location of my choice (within reason). At that time Mobilo would only recover to dealer. Our indy does MOT at a discount - just like MB offering it in the service plan they know they will get unscheduled work from it.
Unscheduled work may involve diagnostics. I may have been luck with our indys but they have not hit us for much in the way of diagnostics. charges (it has typically been free). Our experience with BMW was they wanted paid just to glance at the car.
Back when I had a MB and BMW they'd 'offer' courtsy cars. Unless you booked well ahead they were never available. But that was OK you got driven home or put in a taxi. On the surface that can feel like good service - 'free taxi' - except that in reallity for us being only a few miles from the dealer it wasn't actually worth that much.
When it comes to getting the car seen quickly my experience with BMW was that it was 2 weeks to get an appointment. Took the car in with a problem which was resolved (in terms of dealing with symptom) but not diagnosed. Expecting it to fail I mentioned I would drop by if it happened again - oh no - not allowed - I would have rebook it - two weeks again. With my indy if I had phoned saying I had an active issue they'd suggest I drive round and somebody would at least look at it and give a quick opinion.
And then there is unscheduled work. My experience of indys is that with older cars they are more flexible. They'll fix things that a dealer might insist on replacing - they can get good at doing regular troublesome stuff - the ones that are clued up will know how to get things fixed / reconditioned to save money though maybe at the inconvenience of several days delay.
So I mentioned in a post above that you have to factor in your relationship. If you have a good relationship with a dealer or indy then that in itself is worth a lot.
But MB service plans are designed to smooth the excessive costs of servicing. It's not about saving you money. The underlying costs are bumped up to make the service plans look good value. This is not the only sector that does this with consumers. It's all about getting a yield from the customer. And the indys do the same - if MB put their prices up .... so do they.