I purchased (brand new) a 95 540i, and at that time I thought it was the best car I had ever driven. My ex-in-laws were a BMW family. I purchased my 93 500E in May, and in comparison - my 500E is a better car now than the 95 540i was brand new. After three years of driving the 540i, several things started to fail, wear out, lose their "tightness", etc. The in-laws BMW fleet (525i, 735i, 95 740il, 99 740il, and 99M3 all started to show their wear in three to four years - i.e. significant repair issues. As a matter of fact, only the 99 740il is still owned by the in-laws.
I grant that there are few cars that are built, drive and feel like the 500E, and I can tell you that the MB is much more solid, tight, and a pleasure to drive - especially during a high speed run.
BMW is generally a more spirited drive - and too common on the road. For example, within 10 yards of my parking space at work there are 3 M3's, one 528, two Z3's, and a 325 - while there are three MBs in the whole garage - an E420, a CLK 320, and mine. I "danced" with one of the M3s and word got out - no more tango at work.
I will never go back to BMW. Just MHO. Follow Larry's advice and get your mother to drive both and see which she prefers - after all her opinion is the one that really counts.
Make it a Good One!