I used to drive an '87 300E, traded it for a '00 C230K Sport. Some thoughts:

The 300E was a lot smoother driving than the 202 - the inline six is incredibly refined compared to the four banger, and has a kind of turbine-like feel and sound compared to the more agrarian sounding M111. More of the power is in the upper end of the rev range, so the acceleration kind of builds the faster you go, and highway passing is more satisfying than off the line or around town motoring. (the opposite with the M111, which has a lot more low-end torque than the M103, real snappy around town but a little weak at freeway speeds). The ride in the stock W124 is also a lot smoother than the sport package W202, it took a little while getting used to the firmness of the C230. OTOH, the W202 feels more nimble than the W124, and it's easier to toss around than the 300E.

Cabin-wise, the C230 has a more snug, cockpit like feel. It's definitely cozier than the 124, but not uncomfortably so - I'm 5' 9" and have plenty of headroom / legroom. Controls fall closer to hand in the C230, and the extra technology is convenient to have (one-touch start, remote unlock, remote trunk release). My C230 also comes with folding rear seats, which I find to be really useful. The back seat is smaller than the 300E, mostly in terms of legroom as shoulder room in both is really only comfortable for 2.

From a fuel consumption standpoint, my sense is that the supercharged M111 gets a couple/few mpg more than the M103, but it's noticeably more powerful than the old 6-cylinder was. Pure city probably 18 vs 15, highway 28 vs 22. At least that was my experience. The non-supercharged M111 should be noticeably better than that.

Oh, and in terms of DIY - the M103 is quite a bit easier to work around than the supercharged M111, way more space in the engine bay, but whereas I was constantly tinkering with the 300E, in the last 3 years the only thing under the hood my M111 has needed has been a couple of oil changes and the fan clutch.