Road Bike vs Tri Bike

group ride (specially larger groups) - road bike
solo training or group of triathletes - well, which ever one makes you go faster

I do the majority of my riding on one of my road bikes... even if the ride happen to be a solo ride. Group rides-always road bike, solo rides-depends... if I feel like climbing (which I love), definately a road bike, intervals-quite often tri bike.

With that said, I try to get in at least two rides per week on my tri bike; usually an interval session and one medium-hard ride.