You can buy both cryptocurrencies in order to gain advantage of their respective features. BNB may be faster and cheaper to use than Ethereum, but the number of dApps available on it are extremely limited. Ethereum is known for its rock-solid security and reliability, but the high fees and slow transaction confirmation times makes it unpractical for "De-Fi". Depending on your needs, will be the type of cryptocurrency to use within the decentralized world of Blockchain. Businesses and companies will surely find BNB to be a better option than ETH because of its unmatched performance. Individuals, on the other hand, will find ETH to be a better option than BNB because of its decentralized model. This explains why most people trust ETH on top of BNB.
Nonetheless, the decision of choosing one coin from the other (or both) lies on the person himself. As long as both cryptocurrencies have widespread support from people in the mainstream world, they'll survive for a very long time. What matters is not the coin itself, but rather the technology underpinning these cryptocurrencies. Blockchain is what it's all about these days, as it aims to improve our economy for the better. Just my thoughts