The difference between the two is that bitcoin is the main token or currency while etherium is just an alternative coins. But i do think that of all the alternstive coins existing in the market etherium is the best rival of Bitcoin. Since it has been following bitcoins progress in the market.
Totally different in so many ways. Talking about bitcoin being deflationary while Ethereum being inflationary, talking about how one is more of a currency and the other is more of a smart contract platform and so on. One thing is that they both serve unique purposes which make them very useful in the cryptocurrency space and of course, make them keep being on top as they are now if they end up not getting a push from a competition.