Ok, so I made a thought experiment to decide between the two bikes.
Imagine you are in the shop about to pay £5600 for your M20, or £6100 with a range extender.
The salesman says, hey we have this new version of the M20 in. We listened to customer feedback and upgraded the rear mech from SLX to XT. We upgraded the fork from a Fox 34 to a 36, and the shock from a DPS to a Float X, and we upgraded the bars and stem to race face, and also added a free ec7000 computer instead of that little LED box, but otherwise the spec is the same. We also took the range extender, and put it INSIDE the frame. It's the same weight, but has a little bit less power to let it fit in, so it's 180wh extra instead of 252wh, but that weight is distributed throughout the whole frame instead of one spot, so it feels a bit better than the M20 with a range extender. Oh yeah and it's £400 cheaper too and it's called the H15. Or you can just pay for £950 more for the M20 with the range extender if you like - up to you.
When I set out my thoughts like that, the answer is pretty obvious for me at least!