You can use the commands either place.. the difference being when they are called in the game...
anything in the init line of a unit will occur as soon as the mission begins.... and anything in a trigger will occur when that trigger goes off.....
so.. more than likely you'll want to use a trigger, because the pilot will obviously need to be in range of their enemy
I had another thought as well..... taking a look at the threat model in the plane's config.cpp might give you a clue..
i'm not sure.. but possibly the planes give ground targets higher priority in OFP