I don't know why you have a title like that, you never convert your Bitcoin to Eth or your Ethereum to Bitcoin, there's no logic on that, this is the top two coins in the market, any changes in the market will show the impact on these two coins because this is two most-watched coins in the market.
I always tell my friends to always have these two coins as their main portfolio how the these two coins perform will always have an impact in the market.