First of all, mazal tov. Allow us to give some halachic advice, not a halachic ruling. (Of course, there is also the important issue of how your fiancée will feel about not getting the nicer ring right away, but since we do not know you and that is not our field, we’ll leave that issue out.) Let us deal with two surmountable halachic issues that your suggestions would cause.
To effect the marriage, the groom has to present the bride with something that he owns that is of value.[1] At first glance, if you will have already given the wedding band as a present, it’s your fiancée’s, not yours. It is true that she could transfer ownership back to you before the wedding. However, since she expects to receive it back soon thereafter, we must deal with the question,[2] when one gives an object to another with the understanding that it will be returned soon, whether or not he has intent for a full kinyan (transfer of ownership). In the final analysis, almost all agree that the transfer is effective.[3] It would be even better if your fiancée would state explicitly that she is aware of the issue and intends to give you the band as a full present or would sell it to you. Since this needs to be done with halachic care, this could be “asking for trouble” and is frowned upon by some poskim.[4]
One could also raise questions about the impression given by a single woman wearing a classic wedding band, which is usually a sign of marriage. This situation touches on a halachic topic known as sivlonot. The details are both complicated and fluid, depending on local practice,[5] but here is the basic idea. In some situations where a fiancé gives his fiancée gifts, there is concern that they may be married already by the time the present was given. This can be for one or both of the following reasons. The giving of the gifts itself can serve as kiddushin (initial stage of the marriage process), which does not need specifically a chupa or an officiating rabbi to take effect. It can, alternatively, be a sign that at some time in the past, there must have been kiddushin (or else he wouldn’t have given her those presents). Based on a variety of factors, as reported already many hundreds of years ago,[6] this concern basically doesn’t apply in modern-day society. However, one could raise the issue that, in our days in many places, a certain type of ring, which we call a wedding band, is indeed a very strong sign of marriage. It is possible that the almost forgotten laws of sivlonot should apply to such a case. So again, it may be “asking for trouble” for an engaged woman to receive from her fiancé something which is normally a sign that she is married.
In truth, we feel that both issues are surmountable, and it is the job of a rabbi to solve problems. However, it is also the responsibility of a rabbi to have things run smoothly so that problems are kept to a minimum and there are no grounds to cast aspersions. Everyone rightfully prefers that his or her wedding process be valid without any questions. So, unless there is some type of unusually pressing situation that we are not aware of, we strongly suggest that you not give a wedding band until the wedding. In case of need, you can discuss the matter with your officiating rabbi, who can decide what to do based on the circumstances.