I am getting ready to order my first round of Epoxy. I have decided on the EX poxy system but have a question about the hardner. 83 or 84?

Looking for guidance.

I have a bunch of old Aero Poxy and pump already, but it is 10 years old and may just use it for fill. I am getting new pump and going with the EZ poxy based on a recomendation by a trusted source.