The DFGT and DFP are fundamentally the same thing, but there are some differences. The GT has slightly more precise, quieter and stronger FFB; plus the GT5-specific functions (RA) and a larger diameter wheel with a slightly better shape.
On the other hand, the DFP has the 200 degree lock and doesn't have the PS button to hit accidentally (which has happened to me many times). Overall, if you have a DFP, there isn't really any reason to upgrade to a DFGT. And that's coming from someone who did exactly that. It works better, technically, but not so much so that it will really make a difference to your driving.