In terms of price stability - Bitcoin is the sure winner
In fact ether's price is more stable than bitcoins. Bitcoin is one of the most volatile cryptocoins (considering only the top 3 listed on
https://coinmarketcap.com/).
You can't really compare bitcoin to ether(eum). Bitcoin is basically a currency or store of value (or even both).
Ethereum on the other hand (ether is just the native currency of the ethereum platform) is neither used as a "real" currency, neither for a store of value.
Ethereum is a platform for decentralised apps (DAPPS) which allow smart contracts to be coded and executed decentralized "all around the world".
What bitcoin is for money, ethereum is for anything that can be coded.
IMO both have a bright future, but they still aren't really comparable. But if your question was if ether is going to overtake BTC in terms of price, then the awnser is no.