The G27 is just a new and improved version of the G25, the G25 is discontinued now. The G27 is going to be better than the G25 but if you can find a G25 for a lot cheaper than the G27, then get the G25. Just be careful with the stick shift on the G25, they can break pretty easily if your too rough with it from what i've heard.
Both are great though, gives real driving simulation.