Paul S (suginami) and J Hidalgo are right. The 3.2 M104 was released in '93. My father has a July '93 build E320 coupe and it is not the first because the early ones here in Oz were badged 320CE for the coupe and 320E for the sedan. Paul is also correct in saying the 3.0 24 valve was not used in the sedan in the US, however much of the rest of the world got the 300E-24 (badged that way too) with the early CIS injected M104 as used in the 300CE from late '89 (which were badge 300CE-24 outside the US). Here in Oz, the M103 and M104 were available at the same time in the W124 sedan.
The 3 litre M104 offered a power increase over the M103 but it was all high in the rev range. Additional torque low down did not come until the later 3.2 M104. As Paul suggests, these suffered the well known wiring harness problem (as well as the other problems such as oil leaks and head gasket failures that came with the M104). For reliability and ease of maintenance, the M103 is probably the better choice (although they too suffer oil leaks and head gasket failures). The CIS injected engines (M103 and early M104) also have the advantage that the ECU can lose power or fail completely and the car can still be driven.
I spend time both driving and doing all maintenance and repairs on both our 300TE (M103) and my father's E320 (M104). Whilst there are similarities, there are many differences. The 300TE is proving a little more reliable despite higher mileage. I spent part of last weekend replacing the water pump on the E320 and prior to that it has needed an engine wiring harness and assorted other repairs. The E320 also appears to have the oil leak that is an early warning of head gasket problems. In fairness though, I have also had to do the head gasket on the 300TE.
From a driving point of view, the E320 offers stunning performance. At least Dad's coupe does with its 5-speed auto and 3.69 diff (I think in the US you got coupes the 4-speed as we did with the sedans). The heavier 300TE with its 3.27 diff can't match the acceleration of the E320. Normal starts in the E320's 5-speed are 2nd gear (as with the 4-speed in our 300TE), but if you use 1st you can easily get blistering tyre smoking launches.
The power delivery of the M104 is not as "seamless" as the M103 though. In the M104 you can feel the points where the inlet cam timing changes and where the inlet manifold tuning changes. The E320 has a very hard and heavy throttle (our RHD versions have a regular throttle cable whereas I believe the LHD versions had electronic throttle actuation) unlike the much lighter throttle of the 300TE. The E320 also has noticeable induction noise (an annoying drone or growl) at anything other than a closed throttle. The 300TE on the other hand offers smooth, silent power delivery only becoming audible in the upper rev range. As an MB tech friend once told me "the M103 was one of their finest engines but they stuffed it up when they fitted the 24 valve head to make it an M104". My overall feeling is that the '90 has better inbuilt "quality" than the '93.
My advice is that all W124s were fine cars (although not MB's most reliable) but the first "update" version (late '89 to '92) were perhaps the pick of them.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.