For the first times my friends and I played, we always played pitched battle/annihilation. This proved to be quite hard for my marines against
IG gunline.

However, we now started rolling randomly, because I think it's more interesting when you need to adapt a bit to a situation you couldn't really prepare for. (In detail, I mean.)
It's especially useful when, like us, you set up the map together, each player randomly placing one piece of terrain after another (a die decided who went first). When you KNOW you'll play pitched battle/annihilation, a gunline army can build themselves a fortress with a killing zone in front of it. With the possibility of spearhead deployment and/or objectives, everybody needs to make the battlefield more balanced, due to the necessity of moving around a bit more.