In a world without salary cap, or where there is not a team imbalance favoring D, I would agree that Dumba is more valuable. That is not the world we live in, though. IMO there is not a clear best answer. If I knew that Addison, or Menell would be an adequate NHL RHD, and not RH versions of Hunt, I would feel more comfortable letting Dumba go, for sure.
In the unlikely even that Suter waives his NMC for the ED, I would probably trade Dumba for the best available C, or C prospect, and protect Soucy. I just feel that we must start committing resources to the C position, rather than standing pat with what we have. I could be wrong...it's just an opinion.
The worst case scenario would be for us to trade Dumba, expose Soucy in the ED, and have him taken. I can live without Dumba on the D corps, not without Dumba and Soucy.