In general, Stacking defense is not as good as stacking protection on primaries. With Cody, who multiplies his defense by the number of clones in comp, defense is better than protection. It's hard to say in general because the more defense they have, the more that defense primary is worth, but the more defense it takes to reduce damage too. In general, the ratio at which they give out protection as a primary compared to health or defense makes it a flat out better choice no matter who you're trying to get to survive, and defense is even worse for non-tanks with no abilities enhancing it.
For maxed out tanks with prot primaries and high prot secondaries, defense mods are better than health mods as a rule of thumb these days, especially with all the %health damage floating around.
Defense calculator:
http://realityskewedgamers.blogspot.com/2016/09/defense-and-armor-calculator-version-2.html