I did not mind Wyatt Earp, but it was not my favorite. In my opinion, Costner's best western was Open Range. Earp is too large of a persona for Costner's abilities, he's just not that great an actor. When he tries to portray subtlety it looks like sulking, moping, or just plain stupidity. Russel pulled off being more introspective, in my opinion.
As for accuracy, neither film fits the bill but I think Tombstone captured the feelings of the time more accurately. Tombstone's script did call for a much more accurate rendition of events but circumstances and editing lost much of that. For example, the actor hired to portray Newman Haynes Clanton was ill so they improvised the cross border raid to establish the Cowboy's faction. They had intended to shoot one of old man Clanton's notorious Bushwhackings. Also, the original script was lengthy and developed many characters more fully (that story really is about several characters). Too bad they did not shoot that film. Both films made far too much of Johnny Ringo. Brocius was considered to be more dangeorus and his humor was dark (which is well portrayed in Tombstone), which in Wyatt Earp he comes across as possibly a bad guy but not really a sociopathic one. He gets a lot of time in Tombstone as he should have: he was a famous outlaw.