I will choose both for investment in 2018 and I'm going to put 60% of my assets on ETH, 40% of my assets on BTC. Why? You should know that BTC price is already high at the moment and probably BTC price will raise in a small amount up to $20k this year, I think it is the best time for ETH price to rise slightly, while BTC price raising slowly.
Well I think its a nice decision to make. Since they are both quality coin and good to keep. Then its better to invest in both coin because it will surely give you a great amount of income. Aside from that if you have been invested in both coin , if ever bitcoin fall down you still have ethereum that will save you from loss and vice versa.
So its really better if you have both coin.