There's only one rare earths mining stock investors should be looking at right now.Rare earths mining start-up USA Rare Earth (USAR -7.01%) stock has been on a roll this past week. Ever since rival miner MP Materials (MP -1.20%) signed its $400 million investment deal with the U.S. Department of Defense last week, investors have been betting that USA Rare Earth might be next in line to collect those sweet, sweet government subsidies for itself.But that's looking like an increasingly bad bet. USA Rare Earth ...