M12 over the M18 every day for me.
I own and shoot an M12 and it's been to Africa with me several times.
You get what you pay for and paying a bit more for the M12 is a no brainer.
It is a quality rifle, the action is superb, smooth, slick, positive and wonderful to shoot, it's an accurate rifle.
I haven't shot an M18 but have picked up and handled a few.
The difference to me is marked. It feels like the cheap rifle it is. Different action to the M12 and no doubt good but just not as nice in the hand.
The stock feels cheap and plastic because it is. Obviously you aren't paying for luxury.
Personally I was disappointed as a Mauser fan but I can see where they are aiming (no pun intended) the rifle and to what market.
It depends what you want from a rifle.
You want a cheap beater rifle that you can abuse and get the job done then go M18.
You want a decent rifle that you can abuse and get the job done, get an M12 with the synthetic stock. It will do the same as the M18 but you will enjoy it and appreciate it more, get a better stock and action and it will feel nicer to own and shoot.
If you want a decent rifle that you can take to the wild and enjoy as a companion, looks good, gives you the warm and fuzzies with a smooth action, enjoyable to shoot and own and handle, get an M12 with a nice walnut stock.
Both are good rifles and there for two different markets.
Both will do what you want, one will do it better and please you more.
But you pay for what you get.