I seriously doubt she will wear or WANT to where the Spencer Tiara, as absolutely beautiful as it is.
Diana is represented in the ring, I think the BRF would be offended if she wanted to remind people more of the "Wales' " marriage. I seriously doubt she would demonstrate such a lack the judgement, plus I am certain she wants to forge her own identity and the Wedding and going away outfit will be a large step in this direction.
We already have an idea of her style, it is unlikely to change once married. She may cover the shoulders more and no more plunging necklines (sparkly black issa dress years ago). She will look great. She is a pretty girl with a great figure and pretty subdued tastes for the most part. I think she will be a classic lovely bride.
I am hoping for the Vladimir Tiara, but as HMQ loves and uses that often, I doubt she is would be willing to let Kate become the figure it is identified with.
Aparently tiaras are often retired for a while and skip generations, so maybe HM, will lend her a rarely seen one. A scroll or floral would soften up the lines of her body and usual straight lines of her dress.
Not a fan of those fringe tiaras although spectacular pieces of work...too "sharp", don't like the swedish button/rosette tiaras much either. Oh well, what's a girl to do????