Fromw hat little I've read about these, I think both are very good, but one is better than the other. I read one article that said CDI is technologically better, but with more problems.

What's been your experience with your bluetecs or CDI's?

As the owner of a BlueTec, I didn't think there was any debate. CDI engines are more reliable, have fewer complex emissions controls, and offer better tuning options. The issue is they are generally older so the cars start to have other age related issues.

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2009 E320 Bluetec - 242k miles and a work in progress!

If CDI are more reliable, then how BAD are Bluetec's? Are they marginally more problematic to where them being newer offsets the problems with older bluetecs, ie, 6 to 1/2 dozen of the other? Or is it a thing that when CDI's have problems they're super expensive to fix?

When you get cars that are now approaching 20 years old it's kind of hard to generalize. The OM648 vs OM642 comparison is going to come down to the specifics of the two potential candidates you're considering.

A CDI with 250K miles is probably going to be less reliable than a bluetec with 60K miles. Maintenance history will also play a big part of the equation.

I've never owned an OM642 Bluetec but there is absolutely no question the emission control systems on these is much more complex with high potential costs and limited ability for DIY'er fixes due to emissions control devices requiring computer coding from Mercedes when being replaced. Adblue isn't that big of a hassle as long as the system is working. The OM642 also has the turbocharger in the V of the engine block, great for spooling response and quick warm-up but bad for cooking hoses in the engine bay. There also seems to be a leak issue with oil cooler seals located in the V of the engine.

The Bluetecs were subjects of the emissions settlement, if you get one that was part of the recall I think there is a 4yr/50K mile warranty that basically covers the entire engine. Not sure if the warranty is transferrable.

The CDI's are much less complex from an emissions perspective but they still have quite a bit of complexity. I have two CDI's in service with our family with 340K and 280K miles, the only Diesel endemic thing I've seen is that the high-pressure fuel pump of both our cars began leaking fuel at around 275K miles. Not sure if this is an age or mileage issue but remanned high pressure fuel pumps are in the $1-1.5K range. CDI's can also have issues with fuel injector seals, there are updated injector clamps and seals that are supposed to fix this issue. The 340K car also has started to have injector issues, I can hear quite a bit of nailing under hard/moderate acceleration. Rebuilt injectors are around $300-$400 per injector.

There are lots of threads on here about both engines, do some searches and reading.

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98 Dodge-Cummins pickup (137K)
13 GLK250 (157k)
06 E320CDI (341K)
16 C300 (89K)
82 300GD Gelaendewagen (54K)

The OP asks:

"Which cars have the fewest problems, bluetec or CDI?"

The question might well be rephrased as:
Which engine has the fewest problems- OM648 or OM642?
Why? Because both engines are Common-rail Direct Injection (CDI)
The "Bluetec" engine is a CDI.

For a given mileage the in-line OM648 may be the better choice.

Having worked on quite a few of both of these in my shop, the cdi is more trouble free and way easier to work on. When I look through my customer's service and repair files, cost per mile it's less expensive to own one of the gas counterparts. Rather it be one of the SUVs or sedans. IMO, there's no benefit to owning a diesel anymore.

I think this is undoubtedly true. I continued to drive Diesels because I loved the low RPM torque however the newer generation direct injection gasoline engines are pretty torquey down low and will make better power at higher RPM. As an added benefit they have much less complicated emissions systems and wider support in the repair community.

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98 Dodge-Cummins pickup (137K)
13 GLK250 (157k)
06 E320CDI (341K)
16 C300 (89K)
82 300GD Gelaendewagen (54K)

I can't speak to any om648 issues but having 2 w211 08 09 e320s and a w210 e300 in the driveway I CAN speak to those. If I were in the market for a diesel E class, I would FIRST AND FOREMOST find a shop to work on it if I didn't want to take all the time to learn to diy everything. I have found that most of the shops around me would MUCH prefer to wrench on korean junk or 30+ year old American stuff than euro diesels with computer management and emissions stuff. The smug dealer even looks at us with contempt. For example I was very angry that EVERY car ready for pickup at the Richmond, VA dealer service area had been washed and vacuumed EXCEPT MY 08 E320. When I asked about it, I was told "oh well if you want to leave it overnight we can get to it tomorrow" but that they would also "detail it" and have it ready by "End of Day" if I wanted to pay for it... F those guys. So get a shop first.

Next find a good car. These are old cars so try to get one with reasonable miles. 100K is ok, 350k not so much. And remember the older model with be less complicated BUT will be older- so while the motor may be rock-solid (E300) the rest of the car is getting old and things are breaking down (motor mounts with magic fairy fluid inside perishing, window regulators broken in the down position, biodegradeable WIRING HARNESSES- I guess so you can throw them in your compost pile or toss them in your salad or something because you know conducting electricity is totally over-rated.)

Next suggestion- set aside some money to fix the things it needs, and WILL NEED! So, for the e320s we have, if I had to do it all over again, I'd figure into the purchase to add about $4K and after purchase, immediately take it somewhere to get the stupid $2500 oil cooler O-Ring repair done, but also the valve cover gaskets, the turbo inlet gasket, get the EGR cleaned out and keep funds ready for the swirl flap motor and the stupid turbo actuator that will fail around 100k like mine, if it hasn't already. Also know that you DO NOT want air ride unless you intend to get that repaired or replaced as well, and realize that at 15 years all the rubber junk used in the suspension will begin to fail. My 08 has the notorious "clunking, popping" front suspension noise and everything has been changed but the struts and torsion bar with attached bushings and end links, which will sadly be next. Until then my wife will not ride in it because the banging and clunking when I brake or turn scares the hell out of her. I don't drive it much because the turbo actuator is out so it's miserable to drive and slower than the e300 on a bad day. I bought it at 80K, so I had a pretty good 10K I guess. Her 2009? Its got 280K on the clock..

Having said all that it's like the guys on Top Gear would talk about from time to time. Owning some cars is like being in a relationship, not quite like a marriage- because over time, that is probably less expensive. But there will be good and bad in MB diesel ownership, like in a relationship, that you deal with over time. Dealing with the bad somehow makes you appreciate the good even more. And I LOVE my 08 e320 diesel. I love the 30+ mpg, I love hitting the pedal on the on ramp, dropshifting and rocketing on to the freeway, the comfort of the ride, and the beautiful looks of the car. But I also know after all these repairs that she's higher maintenance and if I had known to use some prophylaxis, or if MB had done it for me ahead of time if I'm honest, I would have had an even better experience.

Goo Ruck... -J