Go with 28mm. Absorbs road shock better. Lasts longer. Fewer flats. Better stability and braking, especially on wet roads. What many of the pros were riding at Paris-Roubaix, where whimpy tires don't make it through a single day of racing.

Many tires sizes are imaginary. Inflate a tire to 100 PSI. Measure the width. Most tires marked 25mm are actually a 23mm. Most tires marked 23mm are actually a 21mm. This is an advertising scam. Companies want to brag about have the "lightest" 23mm tire. A cheap way to build a light 23mm tires is simply to take a run-of-the-mill 21mm tire, and mark it as being a 23mm tire.

Bike companies assume most roadies are morons. And, that assumption sells millions of dollars of tires every year.