Perfect VS Simple

Hi,
I know the main concept of both perfect and simple tenses. But sometimes, I wonder which one should I use. For instance, when I am trying to have a debate. eg I just talked about something and I want to use it in my next dialog. Not necessarily the very first one! But I want to mention it.

"As I told, ... " or "As I expressed in the previous post, ..."
OR
"As I have told, ... " or "As I have expressed in the previous post, ..."

Would you please explain that which one should I use? And does that depend on circumstances or not?

Thank you very much.