Hi -- The bank loan is obviously cheaper and gives you more flexibility by way of having funds available to also buy your bike, both bikes will become your property bought this way.
The finance deal is more expensive and is specific to your girlfriends bike, which will belong to the finance co. until all payments are made in full, however buying through finance offers more protection for the consumer.
So weighing up both options, I would go for the best of both worlds by borrowing the money at cheaper rates from the bank and then actually buying the bikes on credit cards if possible ? either yours or a relatives. This method would give you added consumer protection.
Then repay your credit card by using the bank loan, you will still own the bikes straight away and have the added bonus of the inbuilt protection of using a credit card.