H!

I got asked about what happens when you play Desertion on a commander that's on the stack. My ruling was that the controller of the commander spell chooses which of the two replacement effects to apply first, and if Desertion is chosen then the creature would enter the battlefield under the control of the player that played Desertion. If the commander's own ability is chosen, then it's owner would choose if it would be put into the command zone or not. If s/he chooses not to, then the replacement effect from Desertion would apply.

Talking about it with another judge, he told me that he thought that the replacement effect from Desertion was a self-replacement effect and thus it would always apply first. I'm not sure it works that way, though.

What is the right interpretation?

Thanks!

The other judge you mentioned is correct. Desertion's replacement effect is indeed a self-replacement, so it will be applied first.

A self-replacement effect is “an effect of a resolving spell or ability that replace(s) part or all of that spell or ability’s own effect(s).” That description matches Desertion perfectly.