While I don't disagree with preferring to stop 9/11, thinking that is was a direct cause of the war on terror or tension in the Middle East is simply unfair. Yes, it was the impetus behind the US invasion of Iraq, but the issues in the Middle East are so much more complicated and much more far-reaching back into history (long before the United States even existed as British colonies) than that, and blaming the US for the social, political, and religious volatility over there is just scapegoating. Catholic Europe and the Ottomans were at vehement odds for hundreds of years before the New World was widely colonized, and the ideological and ethnic tensions in that area existed thousands of years even before that. Stopping 9/11 would have minimal effect on the problems over there.
Anyway...
I would choose option #2 - fight in WWII. In WWI, technology had outrun conventional military tactics, and the resulting lack of experience and wisdom led to wholesale slaughter for practically no gain whatsoever in almost every battle. Not that WWII was much prettier, but at least the leaders had a better idea of what they were doing. Also, WWI introduced a plethora of new weapons capable of killing people in such horrendously grotesque manners so far beyond inhumane that the international community agreed to make them taboo. Finally, WWI boiled mostly down to simple nationalism and imperialism, while WWII was fought over nobler and more important ideals.
In the (possibly not-so distant) future, would you rather:
1. Remain on an Earth that is wracked with overpopulation, social strife, and oppressively intrusive government regulation on every aspect of daily life, but be able to maintain a life with sufficient material comforts and a satisfying social network?
2. Leave the world you know behind you, abandoning friends and extended family to live a hard pioneer life on newly-colonized Mars fraught with never-ending labor and few creature comforts, but free to run your own life the way you see fit?