Pick Hardwiring Cable, Power Magic Pro, Ultra Battery, Battery Pack (in that order)
The Harwiring Cable is a simple 3-wire (Battery, Accessory, Ground) harness so it is definitely the easiest to hide for a clean-looking installation.
Power Magic Pro is a small accessory that allows for a stealthy installation in the majority of cases, although it will vary with each car configuration. PMP is easy to hide as you can usually put it in the fuse box. If you want to maintain access to it (to reach the Parking Mode switch), you can install somewhere out of sight under the steering wheel.
The Power Magic Ultra Battery, in spite of its larger size, can be stealthily installed as well, as it is able to connect to the fuse box (recommended installation) and tucked under the driver's or passenger's seat or glove box.
The Power Magic Battery Pack is harder to hide since it requires to be connected to a cigarette lighter socket.
Also, if you want to use the USB ports of the B-112, you will likely have to keep it somewhere in sight. However, if your car features some inconspicuous cigarette lighter power socket, the B-112 can be both handy and stealthy.