It's all crystal ball. I thought IS-F prices would drop below £10k this year. I was wrong!
Looking at Autotrader, it looks like F cars are holding their value better than their M equivalents,
RCF - 34k V M4 - £28k
IS-F - £16k V M3 £13k
You could argue that it is more difficult to sell an F car due to the fact there are less buyers, but you could also argue it's more difficult to sell an M car because there are so many on the market. Currently, on AT, 397 x M3s and 470 x M4s.