Two completely different movies so it is hard to compare. "Wyatt Earp" with Costner, is a biography of sorts tracking the major moments of his life. "Tombstone" is about the Earps' time in Tombstone and the famous gun battles with the Clanton gang. Wyatt Earp is more involved and complicated. Tombstone is more entertaining. Costner is a better cowboy than Kurt Russell.
Again, two different movies. I like both of them equally
"Hour of the Gun" with James Garner as Wyatt Earp and Jason Robards as Doc is better than both of them