I have to agree with the girls, the knot one by itself is stunning, but the oval one will definitely look best with your lock! Post pics of them togeth once you have it, I am curious to see! 

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I think rose gold would be a better choice than yellow gold. Do you have the lock that is pictured? It has a sort of heart shaped silhouette, so I would go with the heart key. To me, the heart flows better than pairing it with an oval.
I would stick to rose gold.
If diamonds are within the budget, there are a few more choices....
My preference between the ones you have posted is the oval.
Yes I have the rose gold lock pictured.
Here's a picture of what I wear pretty much daily:
Would you be wearing a key with the two circle link necklaces shown in your pic? I'm wondering if the key would be too long and get tangled up with them. If so, then I would go for the ring that matches your lock! But if you have room for a pendant with the other items you wear, then my vote definitely goes for a heart shaped key.