Computer to use with a trainer?

[withm]

OR, am I better off just having the LBS transfer the sensor and magnet from the front wheel to the back wheel?

You can probably look up the specs for your bike computer on-line and see the maximum distance the transmitter can send to the receiver on your handlebars. I don't think a wireless computer can go that far but maybe they have changed since I looked into it. I used to have a bike with a wired computer, with the transmitter and magnet on the rear wheel that worked well, until the wires kept falling out.