I think as mmm-five has stated, it is approaching the point where condition and maintenance are more important than mileage. The way I see it is that a 100k car is around 11k and a 70k car 13-14k however, at both these mileages I would do both the rod bearings and valve clearances if they haven't already been done - that is around a £1500 spend. Your cars are suddenly 12.5 and 15.5k however, if you were to sell on straight away would the 70k car sell for 15.5k? I'm not sure it would as I'm not sure the 100k car would sell for 12.5 k although I think it might squeeze 12k to the right buyer.
I genuinely believe that any well maintained M will always be worth 10k and as they age further the baseline will no doubt increase. What I don't know are the timescales involved.