Upgrade existing units or buy additional units

[OldGray]

At the end of each mission you have a chance to buy either new additional units or if they gained enough upgrade existing. I am only on the first human campaign and usually upgraded the units. The problem is later in the campaign I can not field a full army and have no reserves.
Have started again and having more units seems to help with just upgrading a unit here and there. What style of play does everybody use? What works for you? I am on Normal level and do get a mixture of victories of gold bronze etc. I have read people seem to breeze thru it on highest difficulty level and achieve gold victories. If so, can they share how they achieve this success. Needless to say, this is a great game.