For WOODWORKING, the M12 is more useful, but. M12 Fuel can do 90% of what the M18 can do, the question is whether or not you'll run into that other 10%. Of course, the Fuel (brushless motors) is more expensive than the brushed tools.
Why is it more useful? Because working in small spaces is more important than big power with most woodworking.
In reality, any of the Big 4 (Bosch, DeWalt, Makita, Milwaukee) will do ya fine, although if I were looking at 12v I'd focus on Milwaukee and Bosch. (Full disclosure: I have Bosch 12v and DeWalt 20vMax, no Milwaukee cordless. I have been so pleased with the Bosch 12v that I just upgraded my drill/driver from the original brushed motor model to the new brushless motor version.)
I'd agree but spring for the XC flat bottomed batteries on the M12, it's so nice to have them stand up rather than lying them flat on the side.