Is that viable, though? If they have both a military and economic advantage, what is the use in not just rushing them down with ninjas or the like?
If they have both an economic and military advantage at the point where youre both contesting for the tower, you're going to lose either way, unless you pull off some amazing turtle macro (which is unlikely even for top players).
Generally if he has a military advantage, you could assume he has a smaller eco (unless its mid-late and there are already mages and ninjas out), meaning you can fall back and use that advantage. Then if you get passives as soon as you have 2 spears, 2 merics, CA, some archers and a wall, thats enough to stop any spearcher attacks. Then mage if he is continuing to build up his army, or ninja if his military production seems to have stopped. You should end up getting both in the end anyway, but if he stop current military production to instead go for higher tier units (mages, merics, giants), then a mage of your own wouldn't be as effective, as your whole eco is in the line of fire of his own mage, meaning that ninja will save you.