I think for swapping you can choose BNB, it is the cheaper one.

But I think you should change the question that which network is cheaper?, I think now almost all the stable coin in many networks. For example, USDT, it's been on BEP20, ERC20, TRC20, BEP2, etc.

What USDT or BNB do you mean to the question? Bnb and usdt are in BEP20, and both of them are in ERC20 too, so I think BEP20 Is the cheapest one.