Which icing would travel better: buttercream or whipped ganache? Gotta drive two hours & temp will be around 78.

Buttercream just for not to long

I would go with ganache,frost it and freeze it the day before,and by the time you get there it will be ready to serve.An ice box wouldn't hurt!

The few times I have traveled with a cake, the best system for me was to pack the icing (buttercream) in a cooler and frost on location.

For starters: Buttercream. However London_Eats made a great suggestion to frost the cake when you get there. That temp you mentioned is the air, not the inside of your car (or trunk?). I would pack everything up in a cooler with enough fake ice to ensure everything will remain cool, and then assemble the cake at your destination. I think you'll be much more pleased with the results that way.

I think buttercream will be more stable. However, you might want to make the buttercream and transport it in a container and ice the cake at the other end if that is possible.

I think buttercream but I would try to keep it in a cooler if you can