I don't think we can talk about diversification anymore, the whole world, both crypto and markets are strongly correlated
if bitcoin goes up, everyone goes up more or less
if bitcoin goes down everyone goes down more or less
there are speculative movements in certain contexts and certain news
The word preferable is subjective. What you see in Bitcoin might be what another person see and same with Ethereum. Investment wise, I will go with the two because Bitcoin makes moves when you least expect but ethereum moves when Bitcoin moves and moves often Bitcoin market is good, so the two are good investment vehicle in my opinion.
However, technological aspects, they are not even related. It's very easy to manage fees on Bitcoin than in ethereum when there is network surge. Ethereum network is also smart contract compatible, this is what Bitcoin doesn't have so anyone interested in using the network must have a preference and that's why I think technologically, the two are subjective.