Who do you think gets the better end of the deal?

If neither of you have what you're getting, sounds like a win-win. That's the point of trading - not to beat the person you're trading with, but to make your team better, and make you happy (filling the dex, etc.)

I would be happy with both of those things, so I agree with aeronaut in that it's a win/win situation.

I don't think there will be anyone that want relicanth, i believe it is just to fill the pokedex

You get the better deal, hands down. You're getting access to a legacy Pokemon with a ridiculously good moveset, potentially the best rock-type weapon in the game currently aside from SD Tyra.
Meanwhile, he's getting a low CP regional that isn't particularly useful for much aside from novelty. I assume he's going to have to grind for extra candy to power it up anyway.

The legacy omaster is the better end of the deal as is already more rare and is unlikely to be available again.

Conversely the presence of the fairly common double rock ttar means the double rock omystar is now more an aging curiosity then the rock star that it was.