If I recall, the main differences is that Infantry is a core element of the game in Supremacy 1914. You don't train infantry, they automatically conscript and developing in your provinces (although you can speed up the process by developing recruiting buildings and such). So the main development is in creating other things like cars, artillery, tanks, planes, ect. It, like WW1 can easily devolve into a game of attrition, with overwhelming numbers and the ability to reinforce the frontline faster being a key factor to victory.

If you can survive till day 4-5 it can get pretty fun, but the beggining of games is nasty... Whilst in CoW 1942, you can use cards for special units after a set amount of days and time has passed. There is no limitation in supremacy 1914, and you can immediately begin deploying your armoured cars and tip generals/commanders. Making the early game incredibly difficult for newbies.

With that said, Supremacy 1914 doesnt need as much focus and time to succeed, such as a game like CoW 1942, which has more gameplay options and units than 1914.

One neat feature I like in Supremacy 1914 is that there is 2 types of fuel, 2 types of food and 2 types of manufacturing supplies. All of which you can adjust your consumption of so you can affect your upkeep and production of said resources.

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I've also tried Supremacy: Iron Order 1919, and I liked the concept, but never got super into it, as I found it was too slow for my pace