I have a ton of both M12 and M18.  Use M12 mostly around my house and M18 around our farm, but have some cross over at both places.  You'd be surprised how much work the M12 drill and impact driver can do.  Have built fence and deck using them and they are great.  Way less weight but a good amount of power and longevity per charge.  They are also great in a woodshop setting over the M18 due to size and weight.  However, for heavier work like farm equipment, or the F250, the M18 line wins.  Like any tool system, once you amas a collection of batteries, you are married to the tools due to the batteries.  
My prior tools were Makita.  Happened into HD one day that the Milwaukee rep was there and they just made me killer deals to switch systems.  I think I bought 1 tool/battery set with the hammer drill and impact driver with battery and charger.  Then I bought another 4 or 5 bare tools.  Walked out with around 6 or 7 free batteries that the rep kept throwing in.  Long story short, they knew what they were doing because I'm married to the tool line now.  LOL