I think there is alot ot be said about the guy going out and getting a ring to propose to her with. The wedding ring, yes, they should both pick out. But the surprise of having an engagement ring at the time you propose is worth it to pick it out ahead of time. If you are asking you to marry you, you should know what she likes.
Back to the question, I would do it in person at a store. The people who work at ring and jewelry stores tend to know a thing or two to help you out.
If you are concerned about being talked into something outside of your price range, find a store that only hires non-commission employees.





