GamesCom is every year. Celebration is once every 2-3 years. And you choose to go to GamesCom? Odd choice.
Celebration is also of little interest to gamers, but a thing for star wars fans. Personally, as a not-so-much star wars fanatic, I never even heard of it. Whereas GamesCom is one of the major gaming events and will offer SWTOR great publicity to gamers worldwide.
It's not hard to understand why they'd pick GamesCom.