Wow what a great community response - thank you all so much for your replies!
To add a little context and clarity, I currently have a 335d xdrive so the M3P would be my preferred choice to feel like I was upgrading in terms of performance.
Also I've heard they come with indicators which, sadly, the BM did not
The model I drove at the dealership was definitely a 2020 M3P - I wanted as close a comparison as I could get to the 2019 model so consciously made that choice with the demonstrator.
I recently changed jobs and now have the option to charge at work for free so the concept of no road tax (for now), no servicing and free electric to some degree, prompted this enquiry.
Daily round trip commute would be 35 miles or so - I'd expect to charge once a week for those kind of miles and top up at home for any additional trips.
My thinking is that while there is some value left in the BM it may be wise to sell and join the EV shift. That being said, something playing on my mind is the value of the M3P once the remaining drivetrain/battery warranty has expired in just under 4 years time.
I've read so many comments elsewhere advising potential buyers not to touch them once they're out of warranty. I'd expect there'd be no sale value in it by that point, whereas the BM would have substantially more. But then the Tesla would have saved me around half of that value in fuel costs, road tax and servicing... I expect this is a common conundrum for most people looking to cross over that never really reaches a proper conclusion.
As regards considering a newer used model, the M3P seems to rise quite steeply for a 2020 model of the same spec/mileage and I'd wanted to not pay a lot over the reclaimable value of my existing vehicle, otherwise I'd likely go for it.
One last question I do have though relates to the battery.
On the model I'm looking at with 20K miles on it, I asked the dealership salesperson to check the HV battery wear. I think it came back as about 7.5% worn.
Based on what I've read could I assume that's as much the age of the battery, as the number/level/intensity of charge cycles? Is that a good wear level for the age?
I'd still like to be able to take the odd lengthy trip and I'm aware my route/charger planning will need to be a lot more considered as a result. Could anyone advise what kind of real world range I'm looking at on a full charge for both summer and winter months at that wear level, in their experience, assuming a conservative driving style as would be normal for a longer trip?
Again, really appreciate the very helpful comments so far everyone