It depends on where the ASA is being deployed. Most deployments involving IPv6 are done at the edge where routed mode is most natural. The ASA would be deployed as the gateway between inside networks and the Internet, and it can also NAT between IPv4 and IPv6 worlds very effectively. You can also deploy it in a dual-stack mode, if your ISP supports IPv6 addressing directly. If you deploy the ASA in transparent mode, you still need to bring an edge router.
For questions like this, I suggest you head over to our main support community, where you can hear directly from customers: https://supportforums.cisco.com/