The IDE is an all in one Package for developing in ReCT.
It includes a Texteditor, Compiler, Syntax highlighting a Project System and more.
If youre starting out with ReCT this is probably what you should go for. The ReCT IDE is a usefull tool for any ReCT developer no matter the skill level.
The Standalone Compiler only includes the ReCT compiler itself. That makes it lightweight and nicer to use portably. Because RCTC doesnt come with a Texteditor you will need some external one which makes RCTC great to use with Vim or VSCode.
(RCTC is currently the only crossplattform version of ReCT available)