My two cents:
I faced a similar choice. In the end what swayed me was the deal price on my M135i which gave me a budget of £8k to go play with mods with what I would have had to splash out on the 1M. I did have to accept that the M135i was not going to be a looker though, no matter what I ended up doing to it.
If you intend to drive what you buy, either hard or over a lot of miles, then in my opinion forget about words like 'investment' and swap them for 'maintenance' and 'depreciation'.
Having said that the 1M is out-of-the-box-snarly-agressive-manual-fun whereas the M135i is comfort first, sport second but just begging to be modified to tap its potential.
If you're looking for raw driving pleasure the 1M is one of those limited edition cars that will be a gem for a long while, despite it being a chimera of best bits here and there borrowed from 1 and 3 series parts bins. For me it feels dated already though.
Whereas if you're looking for a project and a commuter vehicle in one package then take it from me the M135i is both a nicer place to be in traffic and yet still has plenty of poke as standard. The fun part is making it more than standard
