That's just showing me Forza 2 is using more polygons.
That's debatable. Based on those screenshots, I'd say it's PD that's using more polys. The difference is that they're using them correctly.
Based on the screenshots, it looks like the Forza team is going for an overall high poly count, and that the polys are spread pretty evenly across the entire mesh. While this functions, it's not very efficient. What PD is doing is concentrating the polys where they're needed. Large flat surfaces get very few polys (the hood of the car, for example), while the little grooves and rounded bits get more.
You can see it in the screenshots, and in the game. Check out the screens from Forza 2. It's not hard to spot segmentation in the model. The wheelwells have visible segmentation, and are clearly cut off by a sharp edge. The edge of the boot on the Ferrai Enzo is clearly defined only by a one- or two-poly edge bevel, resulting in more sharp edges. And those are only 720p screens. Whereas I can go downstairs and pop in GTHD at 1080p, and see none of that. Some segmentation on the tailpipes, maybe, but not on the wheelwells.. completely rounded, with rounded edges. Seams along the hood are well-rounded, using five- or six-poly corners.
It's not so much about how
many polys are used so much as it is about
how those polys are used. Comparing those two next-gen wireframes up above, I'd have to say from experience (and I have loads of experience in 3D) that the GT mesh is the better, cleaner mesh.