Heather, question: Do your chicken breasts have the bones or are they boneless? If you are using bone-on breasts (which would be the best way to go here, imo) I recommend cooking them at the higher temp, but to give them about 30 minutes and check the temp. From there, you can probably guage how much more time they'll need to reach 160.
On a side note: Why not do the whole chicken and save the extra for other dishes. The leftovers can be great for a few good uses. Cut the meat into strips to add into a meal-sized salad, or cube up the meat and make a quick chicken pot pie, or even simpler, add the meat into a chinese chicken stir fry or a italian style pasta toss. It would definately help out your food budget in the long haul.