I'd say take it. The 821 is only about 1-2 THs slower than the 841.

Regarding the mention of 3rd party firmwares for s9's - keep in mind that most charge you a couple % usage fee that is taken by the miner periodically changing to what is called DevPool. The Avalon's have no such 3rd party firmwares because they already support extensive built-in performance tweaks to change clock rates, voltage, fans, operating temp, etc.