Im Going To Orlando For The First Time…should I Buy My Tickets Online Or Wait Until I Get There?

Hi everyone. I am currently living in Mexico and me and my girlfriend are going to orlando for the very first time. I want to make our first trip memorable, so I am pretty nervous about getting everything planned.
My first question is should I buy theme park tickets online or wait until I reach Orlando? Are there any discounts in Orlando that I might not get online.
Also let me explain a little bit about my trip. I will be going from the 9th of January to the 16th, 2010. This is what I have been planning I would appreciate any suggestions.
Day 1 (Saturday)- Universal Studios
Day 2 (Sunday)- Island of Adventures
Day 3 (Monday)- Sea World and dinner at Pirate Dinner Adventure.
Day 4 (Tuesday)-MGM Studios
Day 5 (Wednesday)- Epcot
Day 6 (Thursday)- Magic Kingdom
Day 7 (Friday)- Should I go to Animal Kingdom or Repeat a Disney Park?
Day 8 (Saturday)- Repeat a Disney Park.
Im 27 and My girlfriend is 23 and we are both very excited to go. I thank you all for your comments and suggestions. Help me make this the perfect vacation.