Both cars cannot be compared, the e9x M3 is completely different from the F8x M3.. Yes they are both M cars, but that's where the similarities end.
The E9x M3 is slow by today's standards, without a doubt - forced induction changed the game substantially but this is a car from a bygone era, therein lies the appeal.. I completely get the pricing has gotten absurd in recent years, I bought mine when they were a relative bargain at the base of the appreciation curve, but a dream car remains that despite logic or rationality.
There is nothing quite like it, it is and always will be the only V8 M3 ever produced. I love the combination of the American muscle car persona with the German sporty feel.. Going fast has its own appeal but to a petrol-head like myself, it's about the sum of theatre in it's entirety and the feeling you get.. The E9x was my dream M car and its lived up to every expectation I had, which is why I can't find myself selling for the Fx M3.
My advice would be to experience both and then make your mind up, different strokes for different folks.