[Griefbringer]

I would suggest that the advance/delay scenario would be best considered as a hasty attack/defence encounter. For example, the attacker has broken through the main defensive line and is advancing to the rear, and the defending side has hastily thrown reserves to stop or slow them down. Thus neither side has had time to build defensive positions. The attacker needs to push on before the defenders have time to dig in.

Assault/defence is more of a WWI style assault against a well dug-in opponent. Also the attacker has had time to prepare well (thus all the infantry and artillery units also on this side start dug-in to protect them from defensive fire initially).

Also, the two settings tend to favour different kinds of forces and tactics for the defending side. The static defensive positions on defence provide plenty of cover, but only as long as the units tend to stay on their original positions (unless you purchase a lot of trenches for infantry to slowly move around). This tends to favour a strong, static line, with mutually supporting positions.

In delay scenario, no static defences are provided, and the units may need to rely more on their mobility for protection - once spotted, they are in dire danger if enemy manages to mass fire on them. Plus if the enemy has artillery, it may be necessary to spread units around - massed troop concentrations tend to make for juicy targets. Thus a mobile defense in depth becomes a good approach - once a unit has been spotted, it can retire to the rear and take new defensive positions somewhere outside the enemy line of sight, allowing it to re-engage enemy again from a hidden position. This retiring can be covered by other units providing fire to slow down the enemy. Occasional counter-attacks may also be mounted to finish off weakened and isolated enemy units. This kind of mobile action tends to favour armoured vehicles, since they have the necessary speed to change position quickly, cannot be pinned and do not become vulnerable while moving in the same way as infantry. Just take care not to expose the rear armour when pulling back!