I was in the same boat earlier this year - I.e. Eisenmann on the 330d, suspension upgrade, etc.
I went for the M car.
I just cannot decide so I'm just going to see what happens and see if my decision is made for me. I'll either enjoy fiddling with mine and realise I've had no desire to change it, or one day a totally mint Estoril Blue M5 will be for sale and I'll think 'Awesome!'.
The sticking point is the £7-8k cost to change into something which 80% of the time is the same as what I have now.

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