You can only install SSDT, it will install the shell of the Visual studio it is written in. I will suggest installing Visual studio 2015 community edition https://www.visualstudio.com/downloads/ and then install SSDT. If you only looking at creating SSRS reports only then just install report builder. https://www.microsoft.com/en-us/download/details.aspx?id=53613 .SSDT is NOT a new version of Visual Studio its a add on feature to be able to develop SSRS using Visual Studio
Here is my take on your question, from personal experience and researching this matter. Think of it this way SSDT's primary use case is, if you only need to develop solutions for SSIS, SSRS, SSAS etc. Although you are using Visual Studio, you are not using the version of Visual Studio like Community/Professional/Enterprise editions which are targeted for .NET Core 3.x or .NET Framework 4.8.x & have additional benefits you may not use. A good question to help decide is this: will you be creating MVC Webb Apps, WPF Applications? if the answer is no, then SSDT will provide all that you need. I wouldn't use Report Build for SSRS, because it lacks much of the functionality you will need to increase productivity and versioning (Team Explorer).
