I chose the single-button ST-R8060 brake shifters my bike. But, you can choose almost any version of them, depending on your preferences. Both the Dura Ace and Ultegra prior-gen double-button brake shifters would work also and give you the ability to control both FD and RD shifts,.
None at all. I debated it a while, and decided to fully commit to Synchro. I love Synchro, so I have never missed the double buttons. Plus, it makes shifting a little easier, because there is only one possible button to push.