Reaver's Row

This battlefield is now banned.

Q. Do I decide the move the friendly unit at resolution therefore after the attacker resolved its When I Attack trigger, or before?

You choose and commit to move the unit when you put the trigger on chain. If you do not wish to move the unit, you do not put the trigger on the chain. But you will effectively resolve and move the unit (if you commit to do so and put the trigger on the chain) after When I Attack trigger resolved.

383.3.a If a Triggered Ability says “you may” as the first part of its Effect, the controller of its source will choose whether or not to place the Triggered Ability on the chain when its trigger Condition is fulfilled.