Servicing is a fairly routine task (checking/changing fluids etc) so any reputable garage will do a good job but try and find an authorised/accredited Mercedes garage (there will be plenty about) as they will be better suited to picking up on any potential issues (which lets face it - Mercedes are riddled with - especially when it rains
).
Another small consideration is when it comes to selling your car, having a full service history from the dealer may be more attractive to potential buyers.
My best tip is to ring around and get quotes from dealers and accredited mercedes garages. Dont assume that Mercedes dealers all have the same pricing structure, my local dealers were charging close to £100 more for a service than my next nearest. Haggle too as dealers are often in as much competition with each other as with "rival" dealers. It may be more inconvenient but again you can play on that to either get them to do a 1 hour service, so you can wait, or maybe even get a courtesy car for the day. The same rules apply to car hire and car purchase, its a highly competitive business!
p.s. Beware - If your car is still under warranty, using a non-mercedes place could invalidate it.