Being from Illinois, i grew up a Cubs fan. When I moved to Arizona (right about the time that the Diamondbacks came to town), I stayed a cubs fan for a few years. However, with no cable at the time, it was much easier to be a dbacks fan, especially since I had attended numerous dbacks games and they were the local team.
That said, I remained a Cubs fan, and went to Wrigley for the first time on a vacation back to Illinois in 2000. It was a day game in August and Wrigley was mostly empty. The fans that were there were blue hairs. I was somewhat disappointed as I had grown up hearing what a great gameday atmosphere Wrigley had. Granted, this was 7 years ago.
In short, GO DBACKS!!! (but if the Cubs win, that's ok too)