I have both a w124 and a w202. They each have their own strengths and weaknesses, IMHO. The C class gets much better fuel economy (27-33mpg), is comfortable on trips, has newer technology and safety features, and is fun to drive. It is adequately quick on acceleration, but not fast. While several hundred pounds heavier than my 87 300E, it feels a little too light and softly sprung for my tastes. With the fold down rear seat, it is more utilitarian than the 300, but feels much smaller inside than it really is.
The early 300E is very quick, a performance champ on corners, feels solid as a tank, and has a reputation as a rolling safety cage in an accident. Newer 124s are not as nimble or quick, but get better gas mileage. (Mine only gets about 20-22mpg) The W124 has older technology, and some issues that need attention: certain years have r12 freon (can't get it where I live); other years had wiring harness issues; etc. It is an aging design, and entropy always wins.
You are comparing 2 different classes of cars which were designed for different purposes. The only way to get both purposes met involves some compromise (petrol efficiency, power, room, etc.). If the W202 power is making you unhappy, maybe a 94/95 w124, would be the way to go - the AC and wiring issues should be sorted out, and even the 400E gets better gas mileage than the early 300s (won't have the handling, though). Or, my brother gets good performance and reasonable economy from his 2001 E320 - it depends on what you can afford. And you do know the strengths and weaknesses of your car, anything else is an unknown. You will have to decide if the extra cash loss from a sale of the C200 and purchase of an 300 are worth the performance gains.
For what it is worth, my wife commutes over very hilly terrain in her C, and grumbles about the lack of power in the C vs. the E. BUT, she loves the economy and utility: the C is the car we take on long trips or daily commutes, the E is mine for errands and fun touring on weekends.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E