This is a step-up for me from a 1987 300E that I have put over 300,000 hard commuter Km's on. Although the cars look identical, their personaliies couldn't be more different. The 300E is quick and agile while the 400E is smooth and powerful.
The fuel economy is very close. The difference being that if you push the V8 it will drop into the middle teens for mpg while the L6 will stay over 20mpg.
Nice touches on the 400 include better sound deadening and a higher level of upholstery. One wonders what MB would have done with this model id they hadn't brought out the 500E at thre same time.