I would be ignorant if I just say that one of them is best because both of the crypto-currencies is showing their caliber in the market. Both of them considered crypto-currencies, they're both investments that people like to speculate and gamble their money on, they're both based on Blockchain technology and they are both decentralized mediums of exchange. BTC is considered to be the first cryptocurrency as well as the most valuable crypto on the market, whereas Ethereum has the highest upside potential of the crypto-currencies which are now established. So it would be an excellent move if someone would invest in both Bitcoin and Ethereum.
No you would be sitting on the fence instead of being bold there is a winner between ethereum and Bitcoin and that includes how it is designed and how valuable it is. Ethereum will probably not be around in the next 20 years but Bitcoin will and it will still be the most popular crypto. 