• If the sending user has a Lightning-only wallet, then they have no choice: they can only pay via Lightning.

For wallets that support both on-chain and Lightning, it would be helpful to the user to default to the option that offers the lowest fees, which in most cases is going to be Lightning.

In edge cases where the on-chain fees are actually lower than Lightning, then the wallet could opt to send on-chain or even present the user which a choice: "You can save X amount on fees, but this transaction will take longer to confirm. Is this OK?"