Well if you're talking purely DPS numbers earth quake is better even with a type disadvantage. But keep in mind it has an almost 2 second dodge window making it easy to avoid if it's defending a gym. Horn is at 0.3 so obviously much harder to dodge. DPS don't mean sh@# if it's dodged. No reason you can't keep both. I would use earth quake for battling and put the horn attack one in gyms. But that's just how I roll ;)
Edit: Optimal offensive and defensive movesets were just added. They say earthquake for both. It clearly has the better DPS but to me the 2 second dodge window makes it practically useless at defending.