They will coexist. They are quite unique. Each has its own function.
The bitcoin is the first popular digital currency, it has many users. The Etheruem is the first fuel for smart contracts.
These two have highest market cap, it is better to hold both. But the ratio of the holding is quite difficult to decide.
After the recent DAO crisis, there is a concern about the Ethereum now. It will take some time to recover.
I think the Ethereum programs/codes need to be audited properly. It should be more roubust in the future.
Some of the big holder of the Ethereum and DAO are developers themselves. Does it indicate they are not very competent?
The programming of software is very difficult, there is always bug. So the software has to be tested throughly.