Actually, Michelin has done some studies where they claim that 25s are more efficient than 23s. I'm not sure about all the technical stuff, but I think I recall that part of the reason is that a 25 will squish down less than a 23, thereby actually having less surface area touching the road (so less rolling resistance). I've just switched to 25s (from 23s) on my single and really like the difference--no slower, and much more comfortable.

As for the original post--something else to consider is that different companies will have different width tires. For example, a conti 25 is closer to a true 23 while a Michelin may come in closer to a 27 (depending on the rim).