See if your LBS will let you try on a 28 tire. If it fits with some clearance, you should be fine. If not go with the 25. Run your pressure lower than you do on asphalt. You're going to have to experiment on that so you don't pinch flat. There are no hard rules on that, just start high and work your way down.
You might also look into file tread (Roubaix style) tires to give you a little more bite.