Emberskyes says:
Hopefully you military folk learned something from those moments and are controlling the high ground.
There are many advantages to holding the high ground on the attack. But there are just as many for holding the low ground on defense. The environment you are fighting in shapes the strategies available to you. The strategies employed leverage different tactics.
Low ground is especially useful for armored companies as you can "hull down" which protects the platform by minimizing its profile and gives it better mobility via jockeying.
Ambushing isn't perse impossible from the high ground, but concealment makes it significantly easier. An easier mounted ambush is a more effective one. This also applies to flanking maneuvers.
In urban combat or MOUT the low ground allows you to better leverage cover.
In short, mastery of terrain is the ultimate free force multiplier.
Finally don't get too bogged down with making the optimal decision. Often simply making a decision puts you ahead of the enemy. "No plan survives contact with the enemy." They are not going to cooperate.
A good wisdom in this regard:
"When you are retreating artillery always falls long. When you are advancing it always falls short."