I think it's pretty much worthless at this point making comparisons between the two games. The screenshots released thus far for F3 are almost certainly bullshots, and very little in the way of screenshots or videos exist for GT5.
Comparisons could be made between GT5:Prologue and F3, however F3 hasn't released yet and as I mentioned already, most early screens for racing games look noticeably different to the final product. The devs seem to add a number of effects, like increasing the AA, adjusting the colour saturation and adding blur and depth of field to make their screens look fantastic.
A recent example of this being Shift. The early screens looked amazing, then gameplay released and while it looked good, it was clearly not up to the same the level of graphical fidelity as the original screens.
