The re-release was my first time watching Avatar on the big screen so naturally I chose to see it in IMAX and I must say that it was a visually life changing experience. It was simply mind blowing! I purchased the DVD/Blu Ray combo pack a while back and it just doesn't do it justice. A+ on the detail!
As for the added footage, it added a lot to the viewers understanding of the plot. Even if it was just a 30 second snippet, it told you why an event happened. I think that you should definitely watch the original first and then the extended that way you have something to wonder about.
Overall, worth the extra money to see it in IMAX or 3D.