Additives. Use the correct coolant for the vehicle - the variance in price between different coolants is nominal and what little possible savings probably wont be worth the potential long term problems of using the wrong stuff.

Not necessarily diesel vs gas.  Engineers use specific coolant mostly dependent upon the metals and other materials in the coolant system (chosen to not cause damage to them).  Use whats recommended and don't mix brands etc as 2 different coolants can be compatible with a vehicle but not compatible with each other!  

Sorry if you missed my answer. It was in the beginning of my comment. The difference are the additives. Without knowing what specific engines you're trying to compare coolant types of,  it's impossible to tell you the exact chemical difference between the different coolants in each. While I understand that some people really want to know as much as they can, it's kind of a rabbit hole to dive into figuring out different concoctions of coolant when you can simply use the correct coolant for each application. If you do want to dive into it then the technical data sheet of all chemicals is listed on the website of the manufacturer and you can look there to compare the additives. Sometimes though it is considered proprietary so you still might not find your answer.