Only a fool or a master mechanic wouldn't be nervous with the Zeniths.
Hear, hear. I'm certainly no expert, but was fortunate enough to work around MB's from a young age. My father was a zone service manager for MB (70's era) who later opened his own independent garage exclusively servicing Mercedes.
The real bottom line - either fueling system can potentially be more trouble prone, and both are hideously expensive to restore. When I acquired my '70 250 as a "fun" project, I didn't think much about "tuning up" the old Zeniths, which unfortunately had been cruelly butchered by the hands of a now defunct local radiator shop. I quickly discovered there was no solution short of completely restoring them, which took several months of my spare time, quite a bit of money and caused some serious frustration, too. (Don't try this when married, folks.) I've worked on many, many MB's, and especially enjoy diagnostic challenges on the later models - yet those Zeniths pushed me right to the edge. There are other issues on the '72 outside of just the carbs, for example, the system governing the distributor vacuum retard and advance needs to be operating correctly, or you'll never get the car running right.
When maintained, either system will provide reliable performance. In its day, the Zenith really was a fine carburetor. The 250 should instantly start in the cold weather with a pump or two of the accelerator, and run perfectly thereafter. They should stay in tune almost indefinitely. I do agree with carpenterman that MFI was the more advanced system in its day, and might be less troublesome overall. If by some miracle not now, it will still require expert care (and wads of $$$) at some point. If you are a capable "do it yourself" type, the Zeniths will require more time and less money, while the reverse holds for MFI.
If you can find one, have the cars evaluated by an oldtimer MB doc. Maybe you will be lucky, and one or the other will have seen proper maintenance, and hopefully remained relatively unmolested over the years. Much of my above mentioned nightmare with the 250 would have been avoided had the carbs not been nearly destroyed by radiator shop goons. FWIW, my father (who was factory trained on the Zeniths) always said they ran forever with little maintenance, until disturbed by someone who didn't know what the fsck they were doing.
He was simply brilliant with MFI too, even on the M100.
One last remark, Webers have no place on the old six. While I can certainly understand the temptation, if you have the time and patience, the car can be made to run flawlessly with the original Zeniths - with better performance, too. (Please don't take my jab too seriously, I like the little downdraft Weber, they are inexpensive and easy to tune. Just not quite as sophisticated or satisfying to run as a good set of originals.)