I say you should run with the 28's I have a similar commute, 6 miles each way with 3 of those miles through a park. I run 700x25 gatorskins and I find them very comfortable and best of all faster than the stock kenda 700x28c tires I had on my bike.
Just use the tire you have and when they wear out get the 32's if you are dis-satisfied with the gatorskins