1. before proposal, you better find out if your partner is ready for wedding. If you know your partner is ready, just waiting for you. Then it's a good chance for you to give surprise.
2. as mentioned in other answers, the propose ring is better something she likes. you can choose her favorite brand, favorite style or her target size of the diamond. For my wife's case, she could still wear that ring for party and others. So it can be used again is better than just for propose.
For the wedding ring, it's good to have the same style for you and her. If you don't want to have exact style, you want think about my style. My wife and I have the same style wedding ring, but my ring has no diamond, but hers has diamond.
Good luck on your propose and I am happy to answer more question when you plan for your wedding.