Both good coin.
But BTC needs fixing.
So I will say ETH , will rise a lot in 2018. But BTC will still be number one.

I choose ethereum, because I think it will be very promising for a long period of time.

Strong grow in 2018 for all currecncies.
Btc to 30k eth to 5k

if I have to choose between bitcoin and ethereum I prefer ethereum periodically, the ethereum market is greater in fluctuation rate when compared to bitcoin. bitcoin markets mostly focus on long-term investments.

Ethereum Fundation is focused on scalability in 2018. When Ethereum can handle 50, 100, 200 transactions per second (and that's just the beginning) its price will sky rocket.

People are not aware how many projects (hint: hundreds if not thousands) are build on ethereum private networks only because public network can't handle the load at the moment. When it can, most of them will move to public network. Imagine that you instantly have 100x crypto kitties apps on the network using ethereum token and creating value. Can you imagine demand for ETH, the fame, the FOMO? Then you'll see how real bubble looks like

2018 for sure is Ethereum's year.

I think if choosing bitcoin is now too expensive and when the price will go down is difficult to guess, unlike Eth, Eth is still fairly cheap and a huge profit opportunity.

I think ETH is the most valuable and most efficient cryptocurrency in the world. They are serving infrastructure for the ICO's and sure, Vitalin...

I choose Ethereum, because the bitcoin network is almost speculative... and I think ETH will be more for long term investments.

Why not both? That two coin will boom this 2018. But for more ROI go for ethereum, since ethereum is still undervalued compare to the price of bitcoin now. But if you want for long term, go divide your funds in that two coins and sell when you are already happy.