My work involves both maintenance/enhancement of existing databases and the creation of wholly new databases. For the former, I'd most likely use SSMS since it is easier to experiment on views and procedures there. For a new database, my preference is for SSDT.
SSDT offers easier methods for linking to source control (Ankh to Subversion), the more so since not all the developers here have the Red Gate toolset (including SQL Source Control). I also find the table scripts made in SSDT are more appealing - as well as the table definition, all foreign keys, constraints, triggers and permissions are in the one script. The use of variables in an SSDT project means that I can deal with environment-specific users in a generic way. I can also have several related databases in a single solution (eg Staging and Warehouse).