got to be clip in. If they're stick on, then surely some pikey can unstick them. Plus, no adhesive is going to last forever in that environment, so after a year or two, maybe longer, they is gonna fall off.
Will they fall off when your landy is a) parked in the road, nice and neat, with no pressure or stress on the deflectors or b) when you're doing 70 when the deflectors are deflecting and being buffeted and stressed ?
(The right answer leads to deflectors being run over as you watch helplessly in the rear view mirror)