If I could go to one event it would be the finals, you will see the best of the best in the world and also get to preview girls that could be the next Olympians. Usually the finals are split over two days with MAG, usually the last day is WAG floor and vault, first day would be beam and bars. I would choose the firat as I love floor.
To put it short: it would depend on what you want to see. Do you want to see certain gymnasts? (I.e. British team) Do you want to see the winners? (Finals) Do you want to see as much gymnastics as Possible!? (Qualifications) Do you want to see MG and WG? (Event finals) Do you want to see Shawn Johnson? (Stay at home, save your money) Do you want to see cool things (Qualifications?)
Do some research http://www.worldgymnastics2009.com/ All the prices are here, a season ticket would be cool if it was in your price range? Early Birthday present perhaps? I have also heard often that even though you are assigned seats at large gymnastic events, it is often easy to move to a better spot if the area is not too full. Something to keep in mind on the day.
Personally if I was going to one session I would choose qualifications (WG for me) you see way more gymnast and it costs less. You still see all the best gymnasts mixed in with lots of others who have some great stuff to show. The best gymnasts don't always make the finals either. You are more likely to find see footage of finals, so you can still see the best stuff a bit later. 12 hours of action, if your bum can bear it!