paymentId makes sense to store as the gateway_txn_id long term, as it's the value we'll need to use for refunds. The hostedCheckoutId is what we use to get the payment status on the donor's return or during orphan rectifying.
Topic on Talk:Fundraising tech/Chat/Ingenico Connect
hHostedCheckoutId is only valid for 2 hours, so we shouldn't need to put it anywhere except the pending DB.