So I'll be the first to 'fess up to the fact that I have both. My SLK is COMAND+NAV, and it works. My C 200 has an Audio 10 HU, so I opted for the TomTom.
The COMAND in my SLK is obviously not the NTG release, which I've heard really good things about as well. I also don't know how cheaply you can get the NAVI add-on for a COMAND, I've heard $1000 thrown around, but not sure..
The high points on the TomTom - IMHO - are as follows :
1] Touch Screen + Voice Control. I don't have lingatronic, so I can't comment on the MB version, but the touch screen is awesome! Voice control is good, but not stellar.
2] Navigation device first, everything else second. This is a pretty key point - whilst there are other really nice functions built in (Bluetooth phone integration, iPod Integration, on board storage), the navigation interface is quick, simple, and incredibly straight forward.
3] Superb routing, and routing options - side by side, the latest generation of TomTom kicks my COMAND's arse in picking the best of complicated routes. (Houston, Tx is a great test of a nav device!).
4] Map Updates are "free" for the first year.
5] Corrections from TomTom are free, and you can choose to use "User" supplied map corrections as well. (for London this is pretty awesome..)
6] Tons of POI's supplied by third parties (for cash) and from individuals (Open Source
) for free. Even one for Starbucks if you want to find 'em or avoid 'em...
7] Safety Camera Subscription. This one has paid for my TomTom hands down. I don't speed in general, but there are a couple of times where I'd have been done without it - 38 in a 30 type of deals.
8] Free O/S and routing engine upgrades. This is a pretty neat feature, and I've see a few really cool routing and bug fixes released.
9] "Integrates" with AUX-IN. This allows you to play music or your iPod through the TomTom, and have it piped into your car audio system - mutes music to read instructions. Nice, and was a purchase consideration for me, but I've honestly rarely used it.
10] Portability - it's nice to be able to swap it between cars. I travel to the states frequently so end up in rentals a lot. Keep in mind that if you do this like I do - Europe and the States, you will end up having to buy additional maps.
11] Add on features like the Speed Cameras - weather, traffic, etc. Tons of "user customizable" bits and bobs - lots free, some you pay for.
12] Not really a selling point, but it runs Linux
13] Traffic Routing - if you buy the Traffic service, routing takes traffic into account. I tried this in Houston, and was not impressed enough to buy it - but that's mainly because I don't spend that much time there, and I know my way around pretty well for my "common" routes.
Obviously I like my TomTom. It's a cheaper solution by far - although I would recommend at least the 5xx series. I think that the 3D navigation display is better than what I get with the COMAND, and it's a heck of a lot easier for me use it.
It's really a matter of personal choice - I agree that the COMAND navigation in the SLK is very usable, and I've not used the TomTom at all in it. I did not really look for navi on the SLK, but I wanted COMAND so I'd have iPod integration - navi was just a nice bonus.
Oh, and yeah, I've used TomTom 6 on a GPS enabled Windows Mobile phone. It's nice, but just about rendered the phone useless - they also tend to lag behind what they are doing on the dedicated devices. Biggest problem I had was that the volume was crap (function of the phone..), and it was impossible to use without a power cord - it ate the battery (again, a fault of the phone).
I guess my advice would be to try a TomTom and see if you like it - in the states it's trivial to return it if you don't. If you do decide to go the OEM route, try and get the latest set of maps you can as part of the deal, and if it does not come with a mapset, keep in mind that's about $200usd last time I looked. If you have a friend with a MB and COMAND navigation, get 'em to let you play with it a bit before you take the plunge.
My personal opinion was the Germans do great cars, the Dutch have the Navi down! They may be catching up in the latest generation, the guys with '09 SLK's are raving about the new COMAND.