Having had a long term relationship with both models, I have formed a few opinions.....

I've owned at least one W115 or W123 diesel with the OM61X family engine for almost twenty years.

I owned a 87 300TD with the OM603 engine for 13 years, and currently have a 87 300D with the OM603.

It's a natural inclination for a person to champion the style of vehicle they own. For each diesel owner, the particular characteristics of the car, and the circumstances of ownership, fit the individual. But... what's right for me may not be right for you, and fortunately we live in a land with choices....

General percieved thruthisms:

1. A W123 is a simpler car.

Well, yeah it is. If for nothing else other than it can be had as a hair shirt version if you choose the 240D with a manual trans and no options. Again, depends upon what level of comfort you are looking for.

2. A W123 is a better built car.

In what sense? Many owners of W114/W115 cars looked at the arrival of the W123 as the end of MB quality. Each successive version is cast as being inferior to the (beloved) previous one. The W124 brings to the table more modern assembly and material ideas, but is that neccessarily bad? W124s are now 21 years old, and I'll hazrd an opinion that they have proven to be pretty durable cars.

3. The OM617 is a better engine than a OM603.

Again, in what sense? In terms of being able to neglect it and the engine keeps on running, well that's pretty easy to determine. But pit a well maintained engine of either stripe against each other and you see that the 603 is smoother, faster, and gets better mileage. Both engines have adequately demostrated an ability to go the distance, with the OM603 being more sensitive to abuse.

4. The W124 handles better than a W123.

Really, there is no comparision. Just as the W114/W115 was light years ahead of the swing axle, kingpin cars of the 50s-60s, the W124 suspension is positively sublime. The W123 was great in it's day, but it's maturity is obvious when driving the two chassis back to back.

5. The W124 is harder to work on than the W123.

Again, simplicity is the W123's advantage. I've been working on my W124s for over fifteen years now, and I'm quite comfortable doing so, just as I am on my older MBs. It's not intimidating, you still have the typical MB logic and pattern of design. Just a few more creature and safety comforts.

Summary:

Personal preference is key, buy what you feel comfortable in. But almost as important is the particular car you find. W123s are as old as 30 years, W124s are slightly over 20. Condition is key when playing in this age bracket. These cars have the potential for long lives, but by 30 and 20 years, most of the 9 lives have been used up.

There are decent survival rates because of the cars inherent robustness, but the quest for a top ten percenter W124 300TD is going to be hard and/or expensive. There were very few imported, and even fewer that have had continous good care. It's very easy to get a turkey, and you should be well versed in the weak areas of the OM603 before going to look at one.

MB imported many more varieties and years of the W123, which makes a search for this model much easier. BUT, you're dealing with a car that is essentially 10 years older than the W124, and has many of it's own little pecadillos which you need to be aware of. Finding a W123 with full documentation and no deferred maintenance is rare, and expensive when you do come across one.

I love my W123 turbo diesel, my wife absolutely adored her W124 turbo diesel wagon. If youre intimidated by the more complex W124, go to the W123. But drive an example of each before making a decision, only then will be able to make your own choice as to which car is best suited for you! Neither is a bad choice.

Coda: Another choice I have not mentioned is the 81-85 300SD. I had one last year and I liked it better than the W123. Essentially the same mileage, with a lot of the W124 comfort features, similar suspension to the W123, and they were big money when new, so consequently seem to show up in better condition than the W123 cars.

Just my opinions, and like certain body parts, everybody has one.....

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles