Outlook Express is a little more simple to use then Outlook. I prefer Outlook for the calendar. One feature OE doesn't have. It's all a matter or preference, try them both out and you decide which is best for you.
Outlook, as Jewelzz stated, is much more advance and has more options than Express. I also prefer Outlook, but for much more than just the calendar and address book. The task pad is an easy way to keep track of up coming events/items that need to be done. The notes feature makes it easy to write yourself a quick note without opening Notepad and saving a file to your HDD. Outlook is much better than OE, and also much more organized.