Most likely you are getting Express because of its lower cost however if you haven't done it yet do some research on internet to see what you will be missing if you don't buy FCP6 (FCP2 coming with several useful software like Motion, DVDstudioPro4 and much more).
Here is a [useful link|http://dvcreators.net/whats-the-difference-between-final-cut-express-final -cut-pro>.
By the way the info in the link I gave you is not new (almost 2 years old).
Things have changed since, but as I said if you do some research you
will be able to compare the development of the two applications from 2 or 3 years ago until now!