Starting using the 3v lithium batteries when I got my M7 back in '07. Couldn't find them locally back then so starting buying them in bulk. Found out they work great in any camera that took two silver oxide S76 batteries. Even ancient cameras like my Nikon F2. Should work great in your M6 and easier to install than two batteries.
I used to use silver oxide but discovered they don’t like cold and I live near Yosemite and like to take pictures in the winter. They stopped working when they got too cold. I switched to lithium which are fine with the cold.
If you want cold weather performance, search out (and pay through the nose) for Lithium.
For normal use, Silver Oxide are excellent. If you just want to fit batteries for the camera's sale, Alkaline will do the trick.
I have been using CN 1/3n Lithium batteries in both the MP and Nikon F3P. My (limited) understanding is that lithium batteries have a longer shelf life. On the other hand there are restrictions when carrying spare lithium batteries on commercial airlines.
Air Passenger regulations effective 1-April-2016.
1. Lithium batteries carried by passengers in their personal electronic devices, whether in their carry-on or checked baggage, are not affected by this new restriction.
2. Passengers must not pack spare lithium batteries in checked baggage. Spares must be packed in carry-on baggage or carried on the person.