After a long time thinking about the change of 23 to 25 tyres on my Madone, I finally took the plunge a few months back and choose the same type and make as my 23's, Continental GP4000S. I run them at the same pressure as I did my 23's, 120lbs and find them just as good. Cornering is more positive, they are just as fast, maybe even faster as rolling resistance is supposed to be less on a wider tyre. As I do a lot of long distance, the extra comfort is really noticable. The only disadvantages I have come across is that they are a bit harder to push into a strong headwind and it is very noticable, this supports the fact that 25mm tyres are not very good at high speeds due to the extra drag but in reverse, and also due to a smaller clearance in the frame, it could be ride over if you get a buckle in your wheel, unless your prepared to carry on riding and wear your frame away, it is this problem which makes me wonder whether to stick with 25's or go back to 23's. Another problem I came up against, was once whilst going up a steep hill, I stopped dead within less than a yard, the problem was caused by a stone getting attached to the tyre and jamming in the front fork due to the lack of clearance, a new one on me, hopefully if I had been going fast the stone would have been sent flying with centrifugal force or dragged through the fork in an instant.
One of the contributing factors for making the change was the fact a lot of pro riders have been changing to them in the Grand Tours.
If you google for the differences, there is plenty of evidence of going for the change being worthwhile.
But the best bit of advice would be to stick with the make and type of tyres that you are using or you may not notice a difference, or buy a make that is reputable to be faster than what you do have. For instance, on my cyclo cross bike, my 35mm tyres are far faster than my Bontrager 23mm and 25 mm all condition tyres, but slower than both my conti 23mm and 25mm tyres.
