Rather than going to your friends for advice, I think you should go to a jeweler who has professional experience in choosing engagement rings. They will ask about her personality and help you find a ring that she would like, and that fits your budget. Also, have you considered asking her to go ring shopping with you; or are you dead set on it being a surprise? If you two go together, even just to look, she can point out rings she likes and you can go back and get it for her later.
"Caring is not an advantage."