Oooh that's a tough one.. I have both lmao. They are both absolutely awesome. Personally I'd say I like the new m12(released about 2-3months ago) the best, it is insanely compact, barely bigger than your hand, and it has some serious power for its size(110ft/lbs). It is so tiny it feels like an extension of your hand. See photo
On the other hand, the M18 is an absolute beast, and still very compact. The first time I used my M18 impact driver I had a cheap bit in it and was driving some screws on mode 3(high torque). When I got done and took the bit out it was as twisted up like crazy. Also, as much as I love the M12 Fuel drill, the M18 drill is def far superior. So I'd say your best bet is to go M18 for drill and impact driver, unless you're trying to save as much money as possible and don't really care about the extra power.
One other thing to think about.. they did just release an updated drill and impact driver(M18).. July release also. But it isn't much power difference between current and new model. I'd go M18 if I could only do one personally. Then you'll have M18 and M12 batteries, giving you more options down the road.
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