Depends if your playing Classic or Retail.
Classic, the drops are random from what their loot table is. So having 1 of each armor type would be beneficial.
Retail uses smart looting, and it only gives you gear that you can actually wear. So stacking the same armor types gears up your team faster since you can trade with others if they were part of the kill.

Overall though I wouldn't base your team comp on this decision though as you'll eventually gear up just fine as is.