I think it's kind of like iPhone and Android - most people probably prefer what they started out with, but they each seem to have some offerings that are better than the other brand - the DeWalt 20V inflator comes to mind. There are some things I really like about Milwaukee, but not enough to completely switch at this point.
Has anyone ever used the
adapters that let you put a DeWalt 20V battery on a Milwaukee M18 tool, or vice versa? They add a little bulk, but might be a good option for someone that wants a niche tool offering, but doesn't want to buy into another battery system.
Here is one that goes the other way.
I love my DeWalt 12V stuff. It is nice an compact and has adequate power for quick jobs around the house. I think Milwaukee has more tool offerings in their M12 line, but I prefer the grips on the DeWalt - to me they are more ergonomic because they do not house the 12V battery inside the grip. There are also some DeWalt chargers that work with both the 12V and 20V batteries.