In general a 25mm tire will have lower rolling resistance than a 23mm tire. The reason is the contact patch is not as "long" and therefore the tire deforms less.
The Continentals are probably a significantly worse rolling tire than your Michelins. Most continental tires tested with fairly high rolling resistance prior to Conti using black chili, now they are more middle of the line. If you look at the chart I linked to below, the 23 version of the Ultra Sports is about 5 watts slower rolling than GP4000s and Michelin Pro Race 3s PER tire. Which is definitely something a person could feel.
I find Bontrager's lower end tires roll fairly well without being prone to flatting and come in 25 widths.
Here is a very comprehensive set of rolling resistance data, look to the column of crr - which means coefficient of rolling resistance, lower is better. (Also, keep in mind the person collecting the data is doing so for the purposes of time trialing mostly, but also as a service to people and companies. So while a .00300 tire may seem like a "high" rolling resistance, it's quite reasonable for a day in day out training tire) :
http://www.biketechreview.com/tires_...sting_rev9.pdf
Al, who does the above testing, is a very competent tester of tires. He uses a powermeter which he checks calibration on, accounts for changes in temperature and pumps them up to the same pressure every time.