Hard wiring has the benefit of mounting the detector higher without having a large coiled cable in the field of view. A friend of mine has a hard wire set up on his trans am and most of the time you wouldn't even know it was there. It also helps to cut down on people following you to leech of your radar detector. So in short hard wire for Stealth, Privacy,Performance.
Ps: Let me know how your setup work its almost identical to what i plan to do.

-Joe