For those still thinking about it, I have both the DF GT wheel and G25. For anywhere from $50 - 100 US less, the DF GT is an excellent wheel. It feels very similar to the G25, and unless you must have the clutch and shifter, you won't really miss anything if you get the GT. While the G25 isn't directly compatible with Prologue, it does work well - you simply have to keep track of a few odd button differences which are inconveniently located on the shifter. The G25 is also an excellent wheel for PC racing, and I've had little or no problems with it on PC.
I have no doubt whatsoever that the G25, 27 and any Logitech wheel made specifically for GT5 will all be equally compatible.