Even there are more than one similarity, there are some key differs that making Bitcoin and Ethereum completely different cryptocurrencies.
Ethereum is more than just a cryptocurrency. It is high developed platform where beside transfer value users could develop their own projects, applications and ideas. Bitcoin is more oriented on storing value and we can explain this comparative with the fact that Bitcoin was the first cryptocurrency and all other newer cryptocurrencies are developed to be better than previous ones. Ethereum and its currency ether had reached the second place behind Bitcoin. Just like Bitcoin Ethereum is also decentralized system which its making it safer from fraud.
As we already said in the text above, Bitcoin was the first cryptocurrency ever, so it is logically that it is more adopted and has greater value and bigger market cap. Ethereum runs on the principle of so called Ghost protocol and we can see advantages in the in block times. Bitcoin block time is about 10 minutes, while Ethereum block time is 10 to 12 seconds. There are some negative parts of Ethereum, first negative fact is that Ethereum has more orphaned blocks. Second one is regarding transactions. Ethereum calculated every transaction based on its complexity while with Bitcoin every transaction is regarded pretty much the same.
The sole concept of developing these two cryptocurrencies was completely different. Most of the value stored in Bitcoin community is owned by the early miners, when traditional mining was still able. While developing Ethereum, huge number of people funded it and there is some prediction that miners will own 50% of Ethereum coins until fifth year.
But just like all other cryptocurrencies, Ethereum and Bitcoin are very volatile, under the constant influence of the market.