I would make every attempt to keep both. Possibly resign Hardy and hit Dez with the franchise tag again.
For the sake of the thread though, if i could only keep one it would probably be Hardy. You wouldn't simply replace the lost production from Dez with anyone, but i really do believe that with the favorable offensive rules that WRs are becoming the new RBs of the league. Most teams have a very respectable X receiver anymore, and those who do not typically have a good slot wr or receiving TE. Just look at the production that came out of the WR position in the draft a season ago. You could see a duplicate of that this season as well. The NFL is becoming over saturated with WRs. Not saying everyone is on the same level as dez but the drop between Dez and the average WR is not as much as the drop between Hardy and the average DE.