It is a constant source of frustration for some people that it is hard to download SSDT or SSDT-BI or you download the wrong thing.

This is an example of some frustration: https://social.msdn.microsoft.com/Forums/sqlserver/en-US/c82354d0-2f88-4...

Anyway, this is now sorted as there is a single page to download SSMS or SSDT:

https://msdn.microsoft.com/en-us/library/mt238365.aspx?f=255&MSPPError=-...

and SSDT and SSDT-BI:

https://msdn.microsoft.com/en-us/library/mt204009.aspx

Feel free to grab Preview or RTM versions!