I will also go with Bitcoin if the only options would be to choose one and no diversification would be available. Generally, because of bigger popularity Bitcoin has. It would be honest to say, that many, who are not familiar with cryptocurrency, knows Bitcoin, but never heard about Ethereum. Only because of that the demand on Bitcoin will always be greater. If we look on ROI, then Bitcoin also win, its price is more elastic and give more opportunity to earn. Another reason would be cheaper-to-use. While it cost almost the same to send both cryptocurrencies, with Bitcoin it is easier to send and receive, while with Ethereum you can send and burn gas for nothing.