The following link to a blog entry about this very subject, and has the pictures of each tiara as well, is an awesome read. Both on why each one is in the running and the history behind it.
Aestheticus Rex ~ A Tale of Tiara's: What will the Queen bestow upon Kate Middleton?
My personal favourite and hope for seeing on April 29th is The Strathmore Tiara. This was the Queen Mother's family tiara and hasn't been seen since her days as Duchess of York. Now apparently it needs to have a *lot* of restoration work done to it, but what better time or reason for that to be done finally?
Not only that, but for a reasonably young Bride, it's perfect!! It's got History, while not being overpowering and is also a lovely design. I think it would really suit Kate and what we've seen of her personal style over the years. Which is simple and not overly fussy.
Of course, all of that is JMO. What does everyone else think?