This is assuming that you know exactly what frame size fits you and both bikes are the same frame size.
If disc brakes are important to you go with the giant. Everything else is upgradable.
If disc brakes are a fad to you and not important, the Cervelo is a better bike. Better hubs/wheels, will roll better for sure. RD/FD are upgradable. Rest looks pretty similar.
Geo -Longer chainstay/wheelbase on the giant. Wont descend and take corners like the R2. Slightly more relaxed headtube angle on Defy. Stack is much higher on the Defy (like 3cm higher) so you will certainty have a slightly more upright position and will have to cut everything down to get lower.
If you have trouble rolling your hips forward because of your back, then the Defy stack 'may' be helpful.
But as I said in the beginning, make sure you get a frame that fits you. Nothing worse than having an awesome bike that you are all stretched out on.
-Sean