It strikes me these are generalisations based on what we might expect - from both directions. It's possible to work up a case for things being better, or for them being worse.

The big difference between Vietnam and Iraq is that they were conscripts in Vietnam, and they are "volunteers" in Iraq. (Though in some cases "economic conscript" might be closer to the truth; and reservists or territorials who find themselves suddenly in a war zone are a kind of conscript).    But whether that makes them more likely to carry out undisciplined atrocities, or to obey (or give) illegal orders, that something which in principle could work either way.

The aspect of all this that should be the focus of attention is whether there is evidence that there is collusion in any atrocities on the part of those not directly involved - more especially from those higher up in the chain of command, but also from fellow soldiers.