2018... still year of BTC... I don't think that Ethereum will beat a beast yet. Maybe 2020-2021 ETC will go up and up and i'm pretty sure btc will fall down before end. save my post and check in 2022
right now bitcoin supply is ~16.8 million while eth supply is around 100 million.
in a couple of years bitcoin supply will be 17.5-18 million while ETH supply will be around 300 million.
the demand for bitcoin is the demand for it as a currency and also as a secure store of value that is decentralized too. so as more people and more merchants adopt it the demand grows.
the demand for ETH doesn't exist apart from ICO creation and at some point people will stop investing in any ICO as the number of scams grows out of hand like what happened with premined coins and ETH will lose that demand.
now a simple comparison:
bitcoin: small supply with a slow small growth + high demand with a huge increase in demand = huge price
ethereum: huge supply with a fast creation rate and
no cap + low demand and mostly hype = dropping price