Every Time.
The ADC: https://i.giphy.com/media/jMusG7sxBqCze/giphy.gif
Support with Relic Shield: If you'd just let me get it, we'll BOTH benefit. Now doesn't that sound-
The ADC: I WANT IT I WANT IT I WANT IT! MEEEE! MIIIINEE! https://i.giphy.com/media/GzdilzByKGBi/giphy.gif
Support: https://i.giphy.com/media/2AKMqM1HLjZPh2lpep/giphy.gif
That dog is evil.
Relic Shield is a perplexing item to me. Why create an item that encourages the Support to take the ADC's CS? It's a needless source of conflict and arguments, and it makes the game more confusing for new players too.
I believe that is my fault, for training the ADC to take the cannon for fear of me last-hitting it without a Spoils of War charge :)