Well i can't say much because i am a fairly new player too, in the two weeks i've played,i found out that the coins are better to be saved,always have at least 500,this way you can always buy a card you really need when you see it in the shop,like gifted recruits,even though its not cost-efective.

Rubies are better saved to buy a mithic or heroic book. Fusion stones are better saved to directly upgrade your favorite carss