I very much doubt Ethereum will overtake bitcoin. It may exceed its marketcap at some point, but it's unlikely to ever prove a long term store of value in the way that Bitcoin will.
The reasons are subtle but fundamental.
First of all, Ethereum is primarily a technology platform, not a store of value. It's an operating system for blockchain apps and business models. As such, the value lies not in the blockchain token (as it does with pure monetary assets like bitcoin) but in the equity that floats on top of it.
That makes Ethereum hugely powerful as a technology but as useful as a chocolate teapot as a store of value because it's so dependent on its feature set. Therefore, it will go obsolete far faster than Bitcoin.
It's no accident that people are speculating on Ethereum 'overtaking bitcoin' at this point in time - it's almost the only semi-mature smart contract chain out there at the moment. But that's all about to change, specifically on 3 counts:
1. Much of the recent hike in price is due to the massive wave of ICOs which are all priced in Ether. When the wave turns around (the ICO founders start to liquidate their holdings in order to spend) then things might not look so pretty
2. Bitcoin is unique. There is only one original, launched on the 9th January 2009 - the 'supernova' of crypto. You cannot improve on that as a store of value through "better tech" unless you invented a time machine. Stainless steel is far more useful than gold, but not such a great store of value. Ethereum is stainless steel. Far more useful than bitcoin, and in far more abundance (there are already 2 chains, never mind all the alternative brands) but is simply an operating system. It therefore gets more vulnerable as an investment with time due to the tsunami of competitors that's about to come washing over the horizon, including Bitcoin-backed versions such as Rootstock.
3. Regulation. While bitcoin is almost impossible to regulate - being a pure monetary token like a gold coin - the equity that floats on top of a smart contract chain is far easier to regulate. All that has to happen is for national trading authorities to hint at regulating blockchain based ICO's and all that value will run for the hills in mass exits.
So, sure, the froth of Ether may well exceed bitcoin's marketcap at some point. But only in the way that electronic typewriters were big in the 70's. They were ok for a while for making people rich, but pure monetary assets such as gold lasted a whole lot longer.
Bitcoin is yer gold.
Ethereum is yer electronic typewriter.
