it's interesting because my feeling owning both is a bit opposite. M5 feels lighter (and it is, this one just over 3700 lbs with a full tank), faster, more nimble. As i said in the video, the 500e does have nicer interior build quality, but you'd never know you're in something different over another 124 from what you see and touch. I've owned multiple m5's (3.6, 3.8 and a couple of e39s) and this is my second 500e. There's a significant difference between the 3.6 and 3.8, which probably sways me a bit, given both of my m5's (sedan and touring) are 3.8. I've tracked my M5's over the past 20 years. I'd love to get a little more sound out of the 500e.
i agree with you on the e39, that is one i miss. My e60 M5 is currently on cars and bids, and i think i'm going to start looking for another clean stock e39 M5 in an interesting color.