You don’t choose whether to affect the targeted creature’s power or its toughness until the ability resolves.

You apply power/toughness changing effects in a series of sublayers in the following order: (a) effects from characteristic-defining abilities; (b) effects that set power and/or toughness to a specific number or value; (c) effects that modify power and/or toughness but don’t set power and/or toughness to a specific number or value; (d) changes from counters; (e) effects that switch a creature’s power and toughness. This card’s effect is always applied in (b), which means that effects applied in sublayer (c), (d), or (e) will not be overwritten; they will be applied to the new value.