I always thought swordsmen were a nobrainer choice because infantry usually wins fights with static points and extra ws and armor makes it easier to maintain the numbers. 4+ armor save can be tough to pass by while halberdiers light armor is almost trivial.

Also since infantry is more likely to get charged than being the charger, it is more likely that enemy is going to attack first. With halberds you really have to pray for Sigmar that your whole front rank isn't wiped out before you can even try to attempt to score few odd hits afterwards. The extra armor of the swordsmen makes it so much more likely that your troops are alive to fight back after the enemy have attacked.

And even on the subsequent rounds, swordsmen have higher initiative, so they are more likely going to attack before the enemy.

It's been over 12 years since I last played the game (7th edition IIRC) , so my knowledge can be a bit outdated though.