Fir or Hawkeye for DC?

If you only have one DC fodder I would consider holding on to it. Some unit might come along which you really like and want to give DC.

I would hold onto it. Those options are not flattering at all and you’ll regret it in the long run. Just like I killed Brazen Atk/Spd 3 from Winter Robin to put on Sacred Memories Eirika. That was an extremely foolish decision on my part. Don’t do what I did.

Depends on if you've ever given other units DC or if this would be the first occasion.
For me, I've only ever pulled Hector once and I recently gave him to Marth because I'm using him for high tier arena. I know Marth isn't first choice either, but I have no regrets, although there are other units who can rock it very well (Myrrh, Zephiel and Amelia are tops on my list).
Ultimately, it's your decision. If you're going for high merges for tier 21, I'd suggest Hawkeye (or A Tiki, but shes better using lightning breath, steady breath, aether combo in the arena) otherwise hold on to it for later use.

My Hawkeye runs DC and I don't regret it. Here he is in all his stat inflated glory. (Earth season with 3 earth legendary heroes and L!Tiki's c-skill boost)

But if I didn't have other reasons for liking him, I might have preferred to give DC to a different unit. His CD4 Aether is great and all but he's been power crept for real since I built him.