If the 400E is a 92 or a early 93 it will be slightly faster than the E420. Minor differences in engine mangagement and whatnot... Not practically swap-able. Go with the car that has the best service history, paint condition, and feels the tightest. BTW, start saving for the 16x7.5 rims with 225/50/16 tires now. The stock 195/65/15's are more than overtaxed on both models.

Hope this helps...Lee