Keep in mind that my comments are coming from a guy with a GPS mount, but no GPS. Something strange about that.
Lynn Keen helped me to tie the Garmin power cable to the map light. I have since helped another list member install the Garmin cable. There are several wires in the cable, but you need only the positive and ground wire; the other wires can be cut, and the entire cable can be shortened considerably if you connect to the map light (fused circuit). I forget the wire colors but someone has posted it before, or if you can't find it send me a PM and I'll look at mine again.
The map light has simple spade connectors. Add matching connectors to the bare ends of the Garmin wires, and connect them, along with the original map light wiring, to the light switch using Y-shaped adaptors available at Radio Shack or wherever. You will need to enlarge the hole on the side of the map light to pass the Garmin cable inside. Be careful not to remove plastic where the map light hooks in.
As regards the original question, I have the (plastic) Garmin motorcycle cradle attached to my BMR shelf with the slotted bracket. No question the Touratech mount is stronger but it's considerably more expensive. Even the bar adaptor needed to mount the Touratech to the shelf is more expensive than the slotted bracket used with the Garmin cradle. The Garmin cradle includes the power cable but it is also available separately if you go with the Touratech. At one point BMR Products didn't stock the cable only but it's available from other Garmin dealers such as www.getfeetwet.com
The Garmin cradle has a little security screw that adds some casual theft deterrence, but not as much as the locking Touratech mount. Security is a relative term when the whole thing is mounted on a BMR shelf that a thief could probably tear out of the dash of the bike. The Touratech mount sure gets more uber-beemer-geek points, though.
I see that the Touratech website gives a warning about the exposed terminals on the Garmin wire (I can attest to that; I let the power cable touch something as I was installing it, and blew my fuse).
I don't see this as drawback for the Touratech mount, though, since the same cable is used for the Garmin mount. My cable came with a little plastic cap, and the plastic Garmin mount has a little slot to hold the cable in place when the GPS is not mounted.