Both are the best in their own ways as bitcoin is the king of cryptos and will remain always on the top and on the other hand eth will rule the altcoin market but I don't see any competition between both of them as bitcoin will always rule the world.
Both are similar, but has varied features which makes each better than the other. As of now many altcoins came into existence, but very few became popular and reached to be talked along with bitcoin. On that basis bitcoin will be the leader for the digital currency network and ethereum will lead the altcoins market.