Day by Day.
Is spot on today. (see April 21, 2006's edition)
In it there is a little (fictional) exchange between HowDea and one of his minions about how all the armor soldiers now must wear makes them less mobile. HowDea (again fictional) looks at a picture and notes how the soldier in the picture looks like the brother of Ralph (i.e. Randy) from the movie A Christmas Story. Armor has a cost to it, and I am not talking monetary (it has that but there are other considerations aside from dollars). That cost is mobility. Now, the mobility cost of armor has come down recently with technology but the cost is not zero.
What is best? Armor or mobility? That is a huge topic and as such is not easily answered and usually the answer depends on the situation. But the assumption I am fighting here is that armor is the ultimate answer is silly. You can up-armor something ad-infinitum but the enemy can always up-explosive as well. The more armor you have the less mobile you are and the easier target you become allowing the bad guys time to aim more precisely.