Usborne is big enough to handle a school that size. They have over 1200 titles, and with a few weeks notice, can have tons of inventory to do a cash and carry book fair like Scholastic. They don't have the posters, highliters, diaries, and other junk that Scholastic carries, so if that is a big attraction for your students, then you are better off with Scholastic.
In a small school setting, the consultant does 90% of the work...set up, selling, tear down, etc. In a school that large, unless there are many consultants in the area, the school would probably need to provide parent volunteers.
Also, I don't know how your Scholastic rep is, but we never see ours when our book fair is running. With the Usborne book fair, the consultant is there to help parents and children locate books that are age and interest appropriate.
My only problem with Usborne is that I don't think they are big enough to do a book fair at a large school. We have almost 900 kids and I can't imagine an Usborne rep trying to handle a book fair that large.