This might not be such a hard decision if this FO hadn't botched the negotiations with Dak to the extent that they did. As it now stands making that decision becomes one that isn't really that difficult. That's not to say that Rush is necessarily better than Dak.
The expense incurred with Dak is a harder one to swallow than other possible expenditures that might have been made, otherwise. Much needed talents were ignored, due to the exorbitant cost of retaining Dak. Otherwise, the FO might have seen fit to welcome Derrick Henry into the fold, when he was readily available.
On the other hand, one must wonder if both deserve to remain with the team, rather than just one. The FO might easily opt to rid us of both. What is to force one to be chosen without the riddance of the other? The FO will quite likely make such a decision for us. In any event, we're simply mere witnesses, rather than being actual participants.