I married my AAA membership in 1976 (the membership itself goes back to 1970). Over the years the membership costs have risen, some services (towing) have been limited, and we rarely use the service any more. Consequently, and especially since Mercedes will provide some things free, I am looking at changing to another roadside assistance plan.
The following [rough] comparison chart is based on two people driving two Benzes. Our major concern these days is getting stuck in the middle of Snowy Nowhere on our annual Xmas trip. Everything else is tame by comparison. [The "Limits" line in the chart is how many times you can use the service in a year.]
Regardless of the service provider, if you call for fuel, remember to specify
diesel and get them to repeat it back to you. Otherwise they'll send gasoline and blame you. (Perhaps they carry both as a matter of course but I don't trust 'em.)
For close-to-home problems, Mercedes' "Sign and Drive" program will take care of a number of worries. Additionally, it's hard to lock yourself out of a Mercedes, so the risk of needing a locksmith is reduced (but not eliminated, I agree). [As a former GM owner, I always carry a second key, having paid the price in the past.]
At the moment, State Farm wins on a pure cost basis. Obviously, you have to have State Farm insurance to add this coverage but I would suppose most insurance companies have something similar.
The various "auto club" plans tend to offer services not in my small list – travel advice and maps and bail bonds and such stuff – very nice if you need it, a needless expense if you don't. We used to use AAA maps a lot but we aren't traveling as much and can get map info on the web.
AARP (American Association of Retired Persons) is for us old farts (age 50+) and so may not do some of you any good – but just wait!

The Road/Tow plan is
in addition to your AARP membership, by the way.
This applies to the US of A only and as always, YMMV. Use what I've done as a guide and develop your own comparison for your country and circumstances.
Jeremy
__________________

"Buster" in the '95
Our all-Diesel family
1996 E300D (W210) . .338,000 miles
Wife's car
2005 E320 CDI . . 113,000 miles
My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970