I started with the Milwaukee M12 and then went Makita LXT 18V because the M12 first generation batteries and first generation drill chuck sucked. Milwaukee got into the RED batteries with improved 18650 lithium cells and they have been great since; they warrantied all my crappy first generation batteries...took 7 years to straighten that out. Over the last 10-12 years I have invested in both...generally I get ONE new tool per year, but have sometimes been gifted a second by wifey.
If I am doing major construction I find myself with the Makita LXT in hand - favorite tools are the 1/2 chuck drills, brushless angle grinder, LED flip light, 1/4 hex impact, "sawzall", and I just picked up the 18V hedge trimmer...its awesome. Next will be a 1/2 impact; there are at least 3 to choose from. Each tool and each battery has been ultra reliable.
Around the house chores or anything on a ladder it's the Milwaukee M12, because it's small and lightweight. Most used by far is the hex screw gun, then the 2457 3/8 ratchet for vehicles, 2415 small angle drill, 3/8 square drive impact, drills are ok for light work, 2438 polisher was a nice surprise, 2420 hackzall is a jumpy turd, 2460 "dremel" is sketchy, but I do love the 0850 vacuum for light cleaning. They do have a workable LED light, but it started a over $100..we'll see as the price drops. The big XC battery saves the M12 system from being an "also ran", but 5 -7 years of battery problems took their toll on my opinion.
If I had to choose just one system, it would be the Makita LXT...by a nose.
Neighbor does HVAC, and went with LXT. I have talked to a lot of Dewalt users and they love those too; I notice guys who worked outside in the cold hung on to their NiCads because they don't suddenly drop the way Li Ions do when cold.
I agree with others on choice; red, yellow, or teal...a nice "first world" problem to have.