Most reviews I've read, including people who own both, tend to prefer the M2. The 1M has scarcity going for it, that's about it IMO. I think the proportions look off. The modern powertrains are better in both M2 variants. But scarcity is what really matters when you're looking at cars that appreciate, which is why the used market is where it is right now, everything is scarce and everything is expensive. If BMW sold 700 M2s in 2016 and shut it down they'd be worth as much or more than 1Ms.
Current market aside, the number of Mk7/7.5 Rs produced is one reason why they're never really going to be worth silly money. Multiple clean examples are going to be available for a long time (unless you desperately need a spektrum color, which the market so far has shown don't really command much premium outside of the hearts and minds of greedy ass dealers). It's why modern Corvettes depreciate like rocks and Porsche GT cars seem to never stop going up. **Steps off soapbox**