I've heard that most users like their G25s over the new G27s, but I don't own either at the moment. I would say get the G27 over the G25 though.
G27 Vs. G25
I've heard from some extreme enthusiasts that the Porche racing wheel has a much more authentic feel to it:
http://www.fanatec.de/html/index.php?id=250&lang=en