Should I put my wider tire in front or in back?
Generally on a road bike it is considered better to run tires of equal size.

There are a few newer tires which are designed to run different sizes front and rear, but they are usually within 2 mm of each other. (Like the Continental Attack/Force combination)

I would consider a jump from 28c to 23c somewhat large.

That said, run the larger on the rear. The higher air volume will make for a more comfortable ride, and the narrower front tire will steer quicker, which makes for more reactive handling, and has a better aero profile, (which may or may not matter to you).

The comfort part will matter though, and I'd say that's a trump card.