The wired type will give you the most accurate instantaneous speed readout, because it only take two wheel revolutions to get a read. A regular gps such as a Garmin or tomtom will also give current speed, but it is not so accurate, because the current speed is measured over a much longer distance than 6ft.
Where the magnet is on the spoke doesn't affect your reading, but it should be closer to the axle so that you can position it 5mm from the sensor on your fork leg.
What it comes down to, is you need a way to measure distance and time, and there are basically four ways of doing this: airspeed, water speed, wheel speed, and gps speed. There is no magical gizmo that straps on and doesn't use one of four above methods.
GPS is probably the way to go. The tomtom in a family car shows instant speed all the time, but doesn't refresh frequently. But it has a wire coming from it to plug into 12vdc.
It is somewhat better than my bike computer, because the backlight is always on which is good for night riding. If I had more money I would put a gps on the handlebars myself... But those motor vibrations could destroy it.
You could look into motorcycle gps units. They will likely still have wires for power, though.
Your request is fairly specialized, I won't argue one is better than the other, but most people including me are ok with one wire and a magnet, so I think you might have to do your own research on this one. It won't be cheap either. Let us know what ya find.
