ODD vs. RAD - What's the difference?

I have a RAD child and she is attached to us. There are two forms of RAD. Since RAD seems to run on a spectrum I find that it varies so greatly. Our Twin B is affectionate, calm (tantrums less than Twin A who doesn't have the diagnoses). They both have difficulty following instructions. The RAD child doesn't want to do it. The ODD child can't do it.