Ethereum hands down.

Don't get me wrong. BTC is the most powerful cryptocurrency used for TRANSACTION purposes.
Bitcoin is like the USD of cryptocurrency, you can buy any alt coin using BTC.
But at the end, that is the only purpose of BTC, an electronic fiat.

Ethereum, on the other hand, can be used as a platform to construct entirely new applications.
Like the trunk of a tree, Ethereum is a platform capable of branching to endless possibilities.

Another huge issue with BTC, especially as transaction volume increases, scalability.
POW frame works well in the beginning, but as more transactions are jammed down the pipeline, infrastructure cost skyrockets.
Ethereum's imitative to switch from POW to POS is a huge leap forward in the context of scaling.