You can theoretically run a 28 mm tyre at a lower pressure than a 25 mm tyre, without getting a pinch flat, which is where the extra comfort comes from ( as well as having a greater volume of air ). Whether or not you find more comfort from a 28 mm tyre or not depends on how similar in construction and compound to the 25mm tyre it is as well. There are other complications to consider as well, it would take all day to list the various pros and cons, and combinations.