To each his own.Not to be too pedantic, but it depends on how you're defining "better."
Admittedly, I've only see Costner's EARP once and it was many years ago. But it has some pretty big weaknesses - not the least of which is a then-39 year old Costner very unconvincingly playing a teenage Wyatt and its pacing.
While perhaps EARP has overall better filmcraft on display, TOMBSTONE is (subjective call) far more entertaining and engaging. It's eminently rewatchable, which I don't think EARP is.