The 400E is about a half a second quicker from 0-60 mph, and about a second quicker in the quarter mile.
The acceleration from 50 mph on up will feel enormously faster.
I suppose the reliability and comfort should be about the same, although most of the 94-95 C280's have the engine wiring harness problem, and some of the 92-93 400E's have a similar problem.
The C class is about a 7/8 version of the W124 E-class in size, and will probably handle slightly better due to its lighter weight. The E-class has about an inch more legroom upfront and a bit more in the back seat. Unless you're a big guy, I don't think you'll find the C class to be too small in the front, though.
The C280 engine is a 2.8 liter version of the M104 engine. The 1993 300E 2.8 shares the same engine. The 1993 300E (3.2) / 1994-1995 E320 / 1996-1997 E320 have a 3.2 liter version of this M104 engine.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".