I now have the Shimano Speed Deck and the Garmin on the same bike. Yes it looks a little fred but I have no intentions of removing the Speed Deck since it shows gear position and provides a separate readout.
The readout from both devices are very close on speed but the cadence is off. The Speed Deck read 10 higher (80 vs 90) and I wonder why? Yes the Shimano computes cadence but I think all of the gear information is correct.