I've owned quite a few detectors over the years and I currently have a RL360 without the laser diffuser installed in my '09 C300. It's a very nice detector and have noticed excellent range. If you do go with the K40, go over all the install details with the installer! I actually let someone else install my K40 in my C class, and they did a good job with the exception of the LEDs. They put my LEDs in the gauge bezel pointing down. In daylight, I can't see them at all and at night, the light is quite blinding! I recommend finding someone who will remove the gauge cluster and install the LEDs behind an indicator, such as the turn signals. That way you can see it fine during the day, won't blind you at night and is totally stealth.
I also own Escort 9500xi and a Solo S3 detectors but I will most likely get an RL360 for my M3, but I'll install it myself!