I had the G25 for a couple of years and I just got a G27. It isn't that much different actually, but its loads quieter, has a MUCH better feel shifter, and has more buttons, which is handy.
But to be honest, I don't see the G27 as an upgrade from the G25, I see it as a G25 2.0... The g27 isn't better, it just fixes everything that was wrong with the G25.
So if you have £300 to splash, get a G27. If you don't, get a G25.