I have a team of guys who use either DeWalt, Milwaukee and Mikita.
They have both ranges, 18v and 12v and I have the 40v along with the other 2.
I am Makita, I have all the 18v and decided to move over to 40v range. Whilst doing that I wanted 12v Milwaukee after being super impressed with them.
My lowdown, if you want compact with decent power it's the Milwaukee 12v. If you want power and decent warranty then Makita. The 40v is pretty amaZing, I do only have the 40v combi and slowly moving my power hungry tools to 40v, such as band saw, grinder. The rest of my tools are Makita 18v and they have been abused to say the least.
I've had a few tools fail on me after more than 2 years (combi, sds,multi tool) of mostly daily abuse and Makita have always returned within a week fixed.
I wanted to go Milwaukee after trialing their 12v range. But Milwaukee 18v/40v is just solid with a great warrenty so I stayed with them.