Depends heavily on the situation.
primarily, if you have a hero like PA or Sniper who benefit a lot from having targets with much lower armor, while also being less susceptible to focus themselves, use it on the enemy.
if it's an even fight mostly, using it on your own team before the fight allows a longer rampup time for the armor increase, thus meaning there will be a much larger difference than if you use it on the enemy as the fight starts.
a good rule of thumb is, whichever team will take more damage in a fight normally should get the ulti, if the enemy would normally take more damage, amplifying that will allow you to nuke them down faster and lose less health overall, if your team would normally take more damage, the extra armor will give you a big bonus in the fight.
other things to take into account are who has more disables, magic nukes, miss chance skill etc.