Dissipation field and No mercy

[Elfonzo]

Both dissipation field and no mercy appear to trigger at the same time if both are on the field which triggers first or does the control get to choose? Obviously this is as it concerns a creature not just a permanent.

[Double-O-Scotch]

Both {dissipation field} and {no mercy} appear to trigger at the same time if both are on the field which triggers first or does the control get to choose? Obviously this is as it concerns a creature not just a permanent.

[Double-O-Scotch]

If they trigger simultaneously, the controller chooses which order they resolve.