The AI will not attack any healthy army in the field. Since it will not bombard it in the open, until it gets flight, that means they are completely safe in the open, horses or not. It is a tough call to say a sword or a horse army is longer lasting. Neither will last long, swords harder hitters, but slower. That is probably a wash, since the horses get to a spot before the new units are there. Either way both will soon be put in the baby sitting or pilage only business. Horse armies pillage much better than swords. The biggest reason for me to not use horses in an army, is they will become knights.