I think both work together and will grow. Personally, I think this year would be NEO and ETH year and I think both are safe investment for long term, I'll add BTC if lighting solves all the issues facing BTC at the moment.  

ETH has potential this year.

I will choose ETH. Based on my research and opinion ETH is a smart contract chain and more business will deploy their business on the ETH and they will gas to power those contracts. So it will make ETH to grow in value. BTC is good but it only server as a store of value.

Bitcoin will still rule 2018 because all altcoins depends on the price of bitcoin for example if bitcoin will rise then altcoins does it too and if bitcoin falls then altcoins will do the same. And bitcoin these passed weeks have lost so much and I can tell that bitcoin will rise again in the next months

According to the forecast, the price is expected to come out of the range and fall to 9.10 with an intermediate target of 9.34. With the fall of the pair BTC ETH, both currencies can both grow and fall.
For us, a decrease in the rate of BTC ETH with a decrease in two pairs will mean that bitcoin depreciates faster than the air, with growth - the air rises in price faster than bitcoin.
Use crosses and correlations to analyze the situation on the market.

for me is ill go for bitcoin because its still the highest price and maybe it will grow more than the other year and etherium will go higher too and maybe it will grow bigger like the bitcoin.

Considering that interest in ICO in 2018 will be even greater, despite all prohibitions, such blockchain's as Ethereum, NEM, Stellar and NEO will grow very much in price.

I think that you need to keep both bitcoin and ethereum about 50 percent each. Bitcoin can still rise in price if its shortcomings are eliminated, but it already has a high cost and will grow slowly. In turn, ethereum has a lower price, but it grows faster, in addition, prospects for growth in ethereum are very good. Thus, their chances are approximately equal and therefore it is desirable to have these two coins.

I am also confident that bitcoin will remain the king of the crypto currency. But Ether can show good growth. So I invest in both coins.