I'd say the HD was certainly the better razor in terms of build quality, heft and shave experience. I don't actually know what blue tips sell for, but from buying/selling not so long ago the blue tip was the least desireable of the three coloured tips. I think I'd put the HD and the red tip probably about the same money.
Anyway ...
We're not talking collectables here. User grade for user grade, I'd say straight swap. So long as both parties are happy and get a working razor that they can each use, carry on.