If you get a chainsaw mill, then you will find out if you like to mill wood. Some people fall in love with milling, some just want to fall in love. If you do fall for it , then you will for sure have a good saw which you can still use if you were to move to a BSM. Some smaller BSMs can't mill very big logs, You would still have the CSM to break the odd larger log down. If you don't fall for milling, then with a CSM still leaves you with a good chainsaw.
However, the BSM option would leave you with a big piece of equipment sitting around waiting to be sold. If you know you are in love with milling, then you already have a chainsaw(s) now. If so, I would get a BSM.
I bought the M7. I did fall, and I am looking at a n LT 35 . But then, I have the space to store all that stuff when not using it, and like to look at all the stuff I have stored.