EPTThe medivac heals faster then a zealot deals damage without +1 attack. However with +1 attack a zealots and stalkers become "medivac piercing". Furthermore, the difference between 3 hits and 2 hits for immortals against marauders is gigantic.
If a terran player gets medivacs in any sort of timely manner you need collossi or +1 attack in order to deal with it properly.
At the same time, +1 armor is also a very large edge against terran like your post suggests. To achieve both +1 armor and +1 attack before your terran opponent can get his own upgrades, I prefer double forge when I can get away with it. Since you have two forges, you can't get the most out of them unless you get a fast twilight council as well, so my two base shape generally goes something like 4 gates, 1 robo with bay, 2 forge, 1 twilight.
By getting charge fairly quickly, you can really punish the terran through the mid game, especially if they went for the increasingly popular tank play.
