What is better in a 1 unit army - 1 red or black dragon

[McSwan]

I noticed dbdov used a single black dragon in his single unit army run through and got to level 43 without having to change.

How do you deal with not being able to heal the black dragon, or does the black dragon take such a small amount of damage it doesn't matter until lev 43?

It would seem that reds are better from healing, but do they get hammered form hero spells, or something like that etc?