Are both sentences OK?
Whatever your decision is, I'll be with you.
Whatever your decision is, I will support you until the end.

Question

Answer

Steve's comment is a good answer.

Answer

Both are correct.  How important is the decision?  I will support you to the end => this can mean something very bad may happen, such as death.  "Whatever your decision is, I will support you."  => this may be more appropriate, not as drastic.