If you wait a year just to attend Pacific, and put off your acceptance to Tufts, you will still graduate in 2011. So it's up to you if you want to start dental school this year and graduate in 2011, or start your dental school career at Pacific next year in 2008 and graduate in 2011.
If you're from the Bay Area, and want to stay in Nor Cal, then by all means put your effort into Pacific.
I know a lot of Californians don't want to leave home, and would rather stay near home rather than suffer the brutal winters of Boston. Trust me, I have been to Boston during the winter and it's brutally cold! Then you will also have to factor in travelling between California & Boston, cost of moving your belongings, etc.
Good luck with your decision.