Simple "bullet" collision detection, use physics or not?

Imagine bullets going out of my spacecraft and shooting other spacecrafts. All happening in a side scroller. How should the collision be handled?

onEnterFrame
if this.x + this.width >= that.x then
blah blah
end

or something with physics.doThisAndThat()?

Have in mind that the body of the spacecraft is not a rectangle, it can be anything, so we need to know when bullet is inside the shape of stagecraft, not inside the rectangle bounding box or png.

Thanks!

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