Here's my 2 cents... apologies for the wall of text.
I have had a G25, a DFGT, and a Fanatec GT3 RS v2 with CSR Elite pedals. The G25 was nice sorta, certainly had great pedals but the wheel didn't last and actually wasn't entirely happy with the feedback at the time. I think the G25/G27 and DFGT probably have about the same sort of motors (gear driven) and "feel", and I long thought it was pretty good but rubbery. I had high hopes that going to a belt driven Fanatec wheel with a load cell brake would really take GT5 to a new level.
Sadly, it didn't, not for me. The load cell is nice, but the more intense "feel" of the load on the brake didn't seem to match that well with where the brakes in GT5 actually lock up - maybe this is a support issue, but I race with ABS off and never felt like the load cell brake made much of a difference. The real thing though was the wheel, which unfortunately didn't feel quite as responsive though it was smoother. It felt like I was waiting longer to detect when the wheels (especially the rear) would start to lose grip and need some correction, or in most cars I was mostly just guessing how much wheel input rather than instantly feeling it. Changing directions is when you really feel a clunkyness to the belt drive. All this meant my lap times were all slower, anywhere from a second to five seconds. It actually made me start thinking I'd lost my touch.
After three weeks with the Fanatec, I went back to my DFGT (with modded G25 pedals) and instantly got all that pace back. The amazing test is at Monaco in the Formula Gran Turismo, with the DFGT I can use the sequential shifter and basically drive most of the lap left-handed, and I'm over a second a lap quicker than with the Fanatec. And even left-handed, I can instantly and *unconsciously* feel what the car is doing and make corrections. With the Fanatec I often ended up in a tank slapper because the wheel was never quite caught up with the car.
One other test - I still have the original NFS Shift and play it once in a while when I want to drive a 911 GT3 RSR that actually sounds like one. Shift has terrible input lag, but I can sort of get used to it on the DFGT wheel. With the Fanatec, it seemed to add a bit more lag, and became absolutely unplayable for me.
Ironically, the Fanatec did make that Corvette C7 time trial somewhat bearable... but the DFGT is hopeless on that event. I wonder if a lot of what works best depends on what cars you drive primarily and of course what game...