Both are the best coins to invest in and currently both are at very low prices which means it's the best time to do shopping.
It's up to you to decide which one to buy but I can tell you some characteristics of BTC and ETH. ETH is the king of altcoins. ETH has more faster blockchain than BTC and it's blockchain is used by almost 90% of alts. who are developed on ETH blockchain and follow ERC-20 standard. ETH is highly sophisticated, most promising & highly demanded coin after BTC.
From a general point of view, Bitcoin and Ethereum differ in purpose. While Bitcoin is created as an alternative to regular money and is thus a medium of payment transaction and store of value, Ethereum is developed as a platform which facilitates peer-to-peer contracts and applications via its own currency vehicle. While Bitcoin and Ethereum are both digital currencies, the primary objective of Ethereum is not to establish itself as a payment alternative which is different from BTC, but to support and monetize the working of Ethereum to enable developers to build and run distributed applications.
In simple words, Both are best, Buy both
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