i can't comment on the stiffness of DFGT nor the forcefeedback...but all three wheels i mentioned have dual feedback motors so they are all pretty accurate
if you compare G27 to G25 though the steering is a bit more smooth on the former and so is the shifting...pretty much the G27 is just a slightly tweaked version of G25 but it doesn't have the sequential shifting (like you mentioned Initial D styled up/down)
oh and another major difference is that the DFGT pedals don't have resistance so it's harder to tell how much force you are applying...the resistance is VERY well done on the G wheels
http://www.gtplanet.net/modify-your-...re-resistance/