Training or promote troops, which is better?

Promoting is definitely better. It's cheaper and quicker to promote, although technically, it's not cheaper..?

Basically, say for example a lvl 8 rider costs 80k food, wood, steel & gas per unit & a lvl 9 cost 100k of each, you would only have to use the 20k difference each time to promote the lvl 8 to a 9. So it is cheaper to promote them over training new ones from scratch.

I personally promoted every single unit I could, it still cost an absolute fortune in resources but much less than if I had just trained that many. Also it took a long while & I went through over a week's worth of speedups... Worth it though.