I'm a Forza die hard but IMO GT5 at least has a more graphically accurate Nurburgring.
I watched that video, the ZR1 Nurburgring lap, and did a lap in forza and Forza's ring appears flattened and straightened out. For example, coming out of Hatzenbach there is a quick left-right-left onto a small staight before Hocheichen. While in the ZR1 and GT5 the drivers have to almost treat this part as a left-straight-right-left because of the size of the corner, in Forza i can straight line that section using a bit of grasscrete on the right hander (something ther IRL but wasn't in GT4 and maybe not GT5). Also, on the right side of the small straight before Hocheichen is a perfect example of the accuracy GT5. While in real life and GT5 it appears to be a decently sized hill with lots of grass and a NASCAR style catch fence above the close to the road guardrail, Forza's guardrail is quite far from the road (watching the videos, it seems to be like this most of the way around the track). Another spot where GT5 and Forza appear miles apart, and GT5 is closer to real life, is between Aremberg and Adenauer Forst. Almost like driving two different tracks. Lastly, and I think most obviously, the Forza kerbs are massive compared to what is seen the the real life video. GT5 is once again much closer.
If I hadn't spent a fortune on my Prestige edition of COD, I might've just gone out and gotten a PS3 to try the Ring on GT5. Something tells me it is like the GT4 Ring, where it constantly such you back because of the sheer experience it is to drive.