25mm tires can have slightly lower rolling resistance vs the same tire in 23mm.
But they also will have slightly higher air resistance.

In real life, it's probably a wash. You won't notice either lower rolling resistance or higher aero drag.
Note that there is no comfort difference if you run the 25s at the same pressure as your 23s. You have to run at a lower pressure to get the added comfort.

I don't think there is any difference in the tendency to flat.

Before you change, what psi are you running your tires at now? A lot of riders run their tires at too high a pressure because they think that higher pressure is faster. It is but only to a point, and unless you're real heavy that point is lower than the numbers printed on the side of the tire. Try following this chart:
http://www.michelinbicycletire.com/m...rpressure.view

When I did (I went from 110psi to 100), the ride got better, I got fewer flats and I couldn't feel a difference in rolling resistance.