An engagement should never be a surprise. The ring can be a surprise, but you’d do good to ask your partners input since it’s for them. Many couples go to the jewelers together to pick out, generally, why they want. The actual purchase can be made later.
My girlfriend, now wife of seven years, put together a Pinterest board of ring styles that she liked. It worked fantastically well… I still got to surprise her with the actual final ring and she was thrilled that it was the style that she liked and enjoyed the surprise.
Perhaps not for everyone but I certainly enjoyed having it be more thoughtful than me essentially clicking the buy button for her. She did, too.