I realize this is an old thread asking how defense is chosen but I thought I'd resurrect the part the thread alluded to on quantifying marching to the sound of the guns mathematically and discuss defensive settings as well.
Here's a quickie, undocumented (but easily understandable) excel spreadsheet based on the (someone thoughtfully shared on WIKI) quantification of how marching to the sound of the guns works. I believe it works for both the offense and defense identically, however, that is an experience based assumption.
I don't often like to use this spreadsheet in a game as it can reduce the tactic's feel to a (yuck) simple mathematical formula. Yet I find the general knowledge in a defense useful to at least quantify and understand so I can now position my defensive corps/officers in potentially mutually supporting positions using my best officers and terrain to my advantage. One example I might give is in the Eastern theatre......a CSA defense line Nelson-Ablemarle-Spotsylvania, especially in the winter. It is behind rivers and with a strategically high rated general in Ablemarle makes reinforcement of its shoulders likely and thus highly defensible. Sometimes that Ablemarle leader might not be Lee but rather Longstreet or Jackson with Lee's bonus! In fact...surprise.... Lee can be the lesser of the three sometimes. Depends.
Down at the bottom simply fill in the data for one example and voila.....the percentage chance to march to the sound of the guns is provided.
[ATTACH]7006[/ATTACH]
As Arsan alluded to, sometimes its better to defend by giving attack orders of incoming attackers. Especially with high initiative generals and troops.
To abandon the choice of who gets attacked to the attacker is bound to invite the attacker to choose the weakest division defender you've got.
True there is a cohesion penalty inherent in sitting on defense with attacking orders given. But a rested army need not be concerned with that.
As Jackson and later Patton both often stated....if the enemy is having to always react to your offensive actions, you are playing the tune both of you get to dance to.....you control the operational flow. I am a strong advocate for offense action in AACW despite inherent defensive fire advantages and also an inherent casualty disadvantage to the attacker. MTSG helps me do that.
I think this is pertinent to your original question on defense and who gets attacked. I think giving offensive orders on defensive positions can sometimes be the best of all worlds in trapping the attacker.