Both of them can be best based on your preference.
Ethereum transaction fee is not that high anymore and it's one of the most preferred coins of many that have been investing in Defi projects because it's needed for the gas.
But BNB, it's also noticeable that it had gained a lot of projects that are built on its network so it has a share too. Well, whether you choose BNB or ETH as long as you invest only what you afford to lose.
I think that's what all you need to think of.
I think currently BNB has an edge over ETH due to Pancakeswap being so cheap and also due to the fact that there is already a huge number of projects running on top of the Binance Smart Chain. I believe cheap and fast swapping services are a crucial element of networks like ETH and BSC. People want to get in and out of projects and switch between them without having to deal with a lot of friction. That friction can be of any kind, may that be security, costs, speed, you name it.
There's no difference between the two with those swaps. I think something like a project that allows those tokens to be sent on their own despite being ran on their networks.
No need for a gas/eth or bnb just to proceed the transaction.
I think that's a better project despite being made on a smart contract of a project, no need to pay for a fee and without having a mainnet.