My dad had a s500 w220 so same engine. Cooling/heating massing seats are awesome!
Its not really the same my car averages 27ish mpg. My dads s500L used to manage about 20mpg round town and 23-4 combined. You can get 26mpg on the motorway though. Yes the CL will have more niggles as its a car that was 100k brand new and shares its parts with the s500. Remember S klasse means "in a class of its own" and its true. My dad has a e270 w211 now and its nice but not as nice as the s500.
The leather will be nicer/softer in the cl500. More toys in the cl. nicer ride. More bang for your buck. nice stitching in the leather, nicer interior. but on the flip side more toys more things to go wrong. Nicer ride but if it has airmatic the ride will be sublime but when it goes not if but when its big moolah. They also have the cool suction door thing so you tap the doors to shut them thing shlick they are sucked closed.
The CL is a S class coupe really. Its not sporty but if you drive the way it wants to be driven rather than the way you want to drive it, it will be awesome. E.g. try and throw it into a corner it will under steer and not do much. If you take the corner wider wait for understeer then push it out of the corner it likes that and you can keep up with smaller cars. Epic cruisers, tired when coming home or need to do a daily work grind takes the misrableness away as your in an awesome interior with a great engine. But this comes with the cost of being more detached than the clk. But the CL feels german through and through. its built like a tank. Engineering is fantastic. Every material you touch or look at is expensive. This isnt the case in the CLK.
The CLK is just a fancy C class coupe IMO. It looks less dated then CL though. Cheaper to run on fuel but maintenance too. Its not that special on the inside compared to the CL but you could say its more focused. No airmatic Im sure so cheaper there too. They handle so so much better though they dont feel wallowy or boaty on a b road. You can give it some stick and the chassis is great. Lots of feel and confidence. Looks nicer too. Eaiser to park as its smaller but CL's at low speed have finger light steering.
So what should you get? Drive both and see. Engine note is awesome but highy silenced. Get an exhaust they sound great.
The 5l v8's are torquey as hell and rarely need redlining to get performance out of them. I think they are decent on fuel as the CL isnt that much heavier than a 350 plus is the engine isnt labouring at all. Thats the best thing you think the 350 is torquey a 5l v8 shows you what "effortless power" is.
Check for snapped springs funny noises and a few other things on the CLK (cant remember now)
Check the car lowers and rises properly on start and stop for the CL. Check the multi cd changer works and check for water getting into it. Check ALL electric buttons, masaging heating cooling dampener stiffing, raise car, tilt sensor off sunblind up rear headrests down. Check kick down works and the car changes gear with no stutter or jerkiness. You shouldnt really be able to tell when its shifting. Engine should idle under 1k. It should start cleanly first time if not CPS is broke only £50 to fix but problem will become worse if you dont change it. Make sure hot air comes out of the vents if not the duo valve is broke. Check the doors suck in properly. Check the glass is the same all round, my dads was double glazed but some people dont put double glazing if it smashes. the engine should be silky smooth you can do the 50p trick. Handbrake is where your clutch would be and your handbrake release is where your right hand usually is. Dont look an idiot like I did the first time
They are great cars and with them being worth so little dont get them serviced off Merc do it yourself. 90% of things are so so so easy to do on merc they are fantastic in someways.
Hope that helps.