Hello,
It all depends what you want to do with it ...
If it's for sending mail ... and nothing else, you can stick with OE, if you want more : you'll need outlook (or something else) !!!
Outlook Express works very well, but is restricted in use. While outlook is part of the office-suite, and besides an e-mail client it works very well together with the other office-functions.
It has certainly a lot more to offer.
I switched from OE to outlook some time ago, because they changed the server on my workplace en installed also Microsoft exchange server ... and now is everybody on our network working with outlook ...