I have these lights, they're great and should be wired to the main beam.
To cause minimum disruption to the loom, I spliced into the main beam wire inside the headlamp unit itself to get the switched power for the lights. I would avoid those guillotine-type connectors, the best way to do it is to cut the main beam wire, strip about 1cm of shroud from each wire, strip 1cm of shroud from the switching lead, push some heat-shrink along the wire, wrap the wires together, solder them, move the heat-shrink it over the joint and shrink it. Take the switching wire out of the headlamp unit through one of the convenient vents. See pic. Fit a spade connector to allow the headlamp unit to be removed in future and complete the wiring. I put the relays for the lights (I assume you have PIAA's own loom) under the toolbox, including the PIAA switch.
The wires to the lamps and the switching lead run with the main loom on the left-hand side of the bike, past the ABS unit and towards the headstock.
