Roll or Bank

A game offers you two fair dice: an $8$-sided die and a $12$-sided die. You must first roll the $d8$ and look at the result. Then you decide: bank that number as your payout, or discard it and roll the $d12$ once — in which case the $d12$ result is your payout, with no going back. Playing optimally, what is your decision rule, and what is the game's expected payout?