Disney floral did my bouquet and I was thrilled with the result! I was able to just talk with Rosie and tell her what I wanted...and she was able to create it perfectly!! I've heard a lot of good things about her.
I think some of your decision may depend on whether you are having an Escape or Wishes wedding.
With an Escape, it's usually too costly to go with an outside vendor for floral. Even though some brides have said that Disney is expensive, with the Escape package they do provide the basics (bride bouquet and groom boutonniere).
With a Wishes wedding it could be that you need to use Disney floral to meet your minimums, or because you are having your ceremony or reception in a park (Disney does not allow outside floral at in-park events).
There are some brides who have used outside vendors for floral and they would be the best to give you suggestions on that.
Hope that helps.