I have owned a 2020 C300 Coupe since May, but last year I was given an E350 Convertible for a few months while my BMW M140i was being repaired after a van went into the back of it.
I personally prefer the C but that's because I've come from the M140i and various other loud, hard riding cars. So for me, having Mercedes interpretation of a "performance" car is really enjoyable, it's much more quiet and comfortable than I'm used to, but still has a surprisingly sporting edge. By the way I put performance in quotes because someone might tell me it's only a C300 not a C43, but I find it very capable - 0-60 in 5.9 seconds and tidy handling that I don't think I'll find the limit of on public roads.
However, I remember owning the E350 well, and on a long drive I did up to Scotland it was superb. But it's a different type of car, it feels heavier and the handling isn't as sharp, even though it's probably still very capable.
My only disappointments with the C coupe are that the air vents aren't illuminated so the overall effect of ambient lighting is much less impressive than the E, and also the E350 had blind spot warning indicators on the wing mirrors. The C does not as standard and I couldn't find any car available with the package that adds them in.
I guess it depends on what you're used to driving and what you want from your next car. If it's comfort and distance covering ability, get the E. If you want a more involving drive, get the C. But both are good all rounders I'd say. I still rate my C highly on comfort and distance covering - compared to my previous cars anyway