Combat sense has a DV of F/2+2. Each hit adds 1 die to reaction for surprise and 1 die for defending against ranged and melee attacks.
Increased attribute (reaction) has a DV of F/2-2. Each hit adds 1 die to reaction. The force must not be less than the augmented reaction.
So IA® will do everything CS does but will add dice to initiative and have other secondary effects for -4 drain. The caveat is that if you are applying this spell to your street samurai you'll probably have to overcast it.
What's the deal? Is that the only trade off? Why is IA® so much better than CS?