You just noted what I'd consider a desirable feature. I'm told GPS signals don't work underwater, hence we don't have 'Garmin' reef maps to lead us around. Okay. But some dive log software will take GPS coordinates and put them on a map in your log. But active GPS functioning drains battery life faster.
Of course it would be a surface only thing, and be turned off while submerged.
Both computers are excellent and I will love it when they implement a sidemount mode for sac calculation. In the meantime, I use the gas used in each tank and calculate the average and then manually input that into my log software(macdive) and tell it I'm diving doubles. That gives me the sac in volume for the total dive but not for segments of the dive. I'd like to be able to highlight parts of the dive for better planning my gas needs on future dives.
Macdive has using both transmitters on their to do list.