I will choose both for investment in 2018 and I'm going to put 40% of my assets on ETH, 60% of my assets on BTC.
Bitcoin will still rule 2018 because all altcoins depends on the price of Bitcoin.
as for now, i would also do both, but would go 60% in ETH and 40% in BTC. it is true all altcoins depends on the price of BTC, but ETH has lost more value and have more potential to grow with the updates