Aren't they two different wheels ?
I saw some comments on websites saying that the DFGT buttons ar not all programmable or configurable in Windows...they are somewhat "dedicated" to Playstations, and even specifically designed for "Gran Turismo 5 Prologue"...You know Logitech's drivers and software better than I, so I could well be wrong (hopefully).
As far as I could understand: the DFP has been the best (speaking of normal people's budgets ) for a long time, but now it is discontinued and being replaced by the DFGT.
At Logitech's top end there is the G25 which has 2 FFB engines, better foot pedals...
Edit: I'm reading now about the DFGT: some say that the red button doesn't work yet (as of 20th August) and it must be used with the old DFP drivers.
At http://wingmanteam.com/latest_software/latest_soft_main.htm the DFGT looks not supported yet.