Stop calling ethereum "the bitcoin rival". No one in ethereum or bitcoin believes it is a rival. Post-national currencies are not zero-sum
Which got me thinking - can Ethereum or any other cryptocurrency be seen as a Bitcoin rival?
Bitcoin vs fiat
First, lets look at how Bitcoin competes with fiat.
Looking at the definition, a rival is defined as "A competitor with the same goal as another, or striving to attain the same thing. Defeating a rival may be a primary or necessary goal of a competitor." and "Someone or something with similar claims of quality or distinction as another.".
When talking about most fiat currencies like USD, Euro, GBP, etc., or hard assets like Gold or Silver, it might be hard to call Bitcoin a rival to those, at least so far. A lot of national and international currencies exist to facilitate trade, government programs, taxes, etc. on a scale where Bitcoin doesn't register yet. Previous metals are similarly used for some trade, as well as store of value, speculation, etc.
While Bitcoin can fulfil similar niches as those currencies, the currency would first need to rise in value a few orders of magnitude to be able to compete on the same scene. In the future, Bitcoin may be seen as a competitor to USD or Gold, but it will probably take awhile. That, however, doesn't stop it from filling in some other niches.
Bitcoin in various applications
While Bitcoin might not compete against Gold or USD, it can still catch the attention of some gold bugs, internet sellers, or the unbankables. Bitcoin might be too small to compete in the primary markets of companies like PayPal or Western Union, but it seems to be catching up in the more fringe markets.
Bitcoin vs altcoins
Bitcoin's most direct rivals would be the various altcoins.
Looking at the current cryptocurrency market, we have Bitcoin at $6.9B market cap, Ethereum at $1B, Ripple at $206M and Litecoin at $181M, with every other coin having substantially less than $100M.
Bitcoin's most direct competitor feature-wise would be Litecoin, sitting at 1/38th of the market cap. While it might be a notable currency for speculation, there doesn't seem to be as much adoption and development push from within and without the Litecoin community to say that LTC is competing with BTC. As such, it doesn't look like a potential rival for Bitcoin.
Ripple, due to the centralized control of its XRP tokens, can never hope to compete with Bitcoin. Similar to Omni and Counterparty, it might be better suited to be a Bitcoin compliment - dealing with user-created currencies, while leaving Bitcoin to be the decentralized currency.
This leaves us with the main topic - Ethereum...
Bitcoin vs Ethereum
Ethereum is a bit of a mixed bag. Its genesis block started with ~72M ETH being created for the presale (~60M ETH), the developers, and the Ethereum foundation (~12M ETH). With the current supply of 80M ETH, that presale constitutes a large chunk of the total ethers in circulation. Some might see that as premining, while others, like myself, don't see similar presales as such.
Ethers also aren't always viewed as currency, but rather as a token for executing code on Ethereum. While that might be true and some core developers might say that for legal reasons (to protect themselves from any legal fallout from the token presale), it hasn't stopped people from speculating on the value and bringing the value up more than 10 fold in the last year.
Lastly, Ethereum does a lot more with its scripting language than what Bitcoin can. Until we get something like sidechains up and running, Bitcoin will probably not be Ethereum's rival anytime soon.
However, the opposite might not be true. Ethereum has 1/7th the market supply of Bitcoin, a large community around the world, and is starting to get high-profile projects like The DAO. As such, Ethereum is shaping up to be a rival to Bitcoin.
With all of that being said, Ethereum and Bitcoin filling the same niche of decentralized internet currency might not be too bad. Both of the currencies still have bigger opponents to overcome on their way up - fiats, precious metals, centralized payment processors taking big cuts, etc. Success of one might not mean the loss of other. As long as both communities remain on good terms, developers, exchanges, and other crypto businesses are open to accepting both currencies, and we keep our eyes on the same target of overcoming the old way of banking, there is no reason why this rivalry couldn't be a friendly coopetition.
Bitcoin is not yet a rival to the big fiats or precious metals, as it is too small to register. Most altcoins aren't big enough to compete with Bitcoin. Ethereum is a potential rival to Bitcoin, but there is no reason for competing directly with one another when there are old currencies and use cases to take on first.