I have both the m18 (gen 1) and m12 rocket lights.. the m18 is the superior model.. its bigger, brighter, more stable.. its just a more robust product if you directly compare the two. The gen 2 m18 even more so, and the gen 2 is dual powered and can be a charger as well if that matters..
The m12 is good if size and space is an issue. Its a little less bright, but a lot smaller.. easier to carry around, store in a vehicle, and its also dual powered but no charger.. there is a big difference in brightness between the gen2 m18 and m12.. the gen2 m18 is also heavier than the gen1 m18.
They are all good. They are all bright, some or just bigger and brighter then others. Its really a matter of what model fits your needs better.. if your not traveling, going in and out of houses, then size doesnt really matter. If your not a mobile professional then having the charger on the gen2 m18 doesnt really matter either..
If smaller is better for you, get the m12 . If it doesnt matter, get the gen1 m18. If you can benefit from the charger in the base, on need it to be dual powered, then get the gen2 m18. If you need to light up a bigger area - then m18. If you need to light up a whole room,and for it to be bright, then gen2 m18 because of the 3 directional heads..
Or just **** it up and buy the m12 and one of the m18 models.. lol
I think the gen1 m18 also got an upgrade too.. i think mine is 2000 lumens, and the newer is 2500.. so i think that makes the m12 1500 lumen, gen1 2500 now, and gen2 3000. But check on that.
Edit - and actually the newer version of the gen1 m18 is 2500 lumens, and dual powered also. (Mine are not).. so the gen2 m18 is 3000 lumen, dual powered also, but has the 3 sided head you can aim in all different directions.