There are (at least) three things to consider:

1. Which tire matches your rims best?
2. How fast do you expect to ride?
3. How rough/smooth do you expect the road surface to be?

I would think you'd want to use a 28 on the rear. I suspect (but don't know) that if the road is smooth, you're fast, and you have the right front rim, a 25 might be marginally faster than a 28. But because having to keep two different sizes of tire is kind of a pain, and sometimes the road isn't smooth, I might run 28 on the front.