you can't really compare these two coins, they are completely different from one another. BTC is the first application for the blockchain which was and will always be, that's quite a big deal because most of its current success is not due to the fact that it's a better network when compared to others, but the fact that it has been the first in the market and therefore the most popular. Bitcoin has went through a lot, Ethereum is still relatively young which means that it might offer some better returns that Bitcoin (if you invested in one of them today) but eventually Ethereum needs to be tested as well, it was only in 2017 that it took off so there is still time to see how things will be handleded both internally and externally.