Hi Mark,
Sorry for the late reply.
My experience with the Mercedes was good - very reliable & pretty good fuel consumption - around 40 mpg roughly. My experience with the Mercedes dealer in Exeter was good & I never had any problems with them. My only real reason for changing was that diesels are becoming increasingly seen as 'unclean' in some areas & I didn't want to start incurring penalty charges for entering various towns in the future. I'm sure Road Tax will rapidly increase for diesels. I do miss the 'cabrio' style though!
I have not driven the Lexus much - the West Country is currently packed with tourists & parking is very difficult so I have been using my Smart car as you can always find a small space big enough to park. The Lexus is much quieter than the Mercedes of course. I took it for a run yesterday with four adults up & it proved very comfortable for all concerned compared with the Mercedes which was decidedly cramped in the back. Its too early to talk about reliability but as I'm covered with a 3 year warranty, 3 year breakdown & recovery, etc., I feel that I'm future proofed for the next 3 years.
Hope this helps - again, sorry for the late reply.