Both are top 2 altcoins in the market, i think you shouldn't hesitate to choose one of the two but should choose both. both have great potential and are worth the long term investment.
Investing in both would be the best thing to do. They're are a blueprint cryptocurrency and shouldn't be ignored or regret in future when they're trading at higher price. I strongly believe both coins can do a minimum of 10x in the next bull market circle. BNB has quarterly burns that would drive up price while Ethereum has the utility every other cryptocurrency project is hoping for. Both blockchain will be used frequently as they're the leading project in their respective fields.
BNB though is the most promising if you as for my personal opinion so maybe investing more in BNB will be great as well but that doesn't mean you should ignore Ethereum. BNB is also cheaper which is why it can go more higher.