Well the current 3 hatch is already safer than the car you already own, i can't imagine the safety features increasing all that much. The current car has front, side, curtain airbags, tpms, etc...etc... You'd probably be able to score a really good deal on an '09 once the new 2010 models start filtering in.
I just look at it this way. Mazda has had 6 years to perfect the current 3 while the new generation will only be on its first. I'd go for the one that's been tweaked over the years. Also the power won't be all that much different. Although i have been hearing reports that the next gen 3 will be 200lbs lighter. So it may 'feel' like it's a much quicker car.
It all comes down to what you want. Yes, you'll get a better deal on an '09 3 but you also have to enjoy how it looks, drives, and feels all around. I would test drive them both back to back once the new model arrives, then you can make a fair assessment.