Does she care about name brand stuff? If so, she might prefer the natural, dug from the ground diamond. Is she practical and likes a good deal on quality? She might prefer the lab grown diamond.
I can't say that I have ever compared the two. Chemically they should be the same, so if you can get more clarity, purity, and sparkle for the money, I'd go the lab grown.
That's what I'm thinking. Good lab grown diamonds are more perfect. You can get better clarity rating and more carat weight for less money.
A friend of hers got his fiance a $10k ring and she told me don't you dare finance a $10k ring for me like he did. She's more practical than that.