Forza's addition of Ferrari and Porsche does make a difference. Forza also did a better job in the graphics, mostly with textures and surface quality, but the cars dont look quite as crisp as those found in GT4. The car's feeling between the games is very different, Forza doing a better job overall with tire wear and weight distrobution (owed to the XBOX's power), but lost out to GT4's attention to detail in the way the cars would react to imputs from the driver.
GT4 may be a superior product in many peoples opinion, and the same can be said for Forza. Be thankful that there is competiton for the title of "Best Sim Racer," because as we have seen with Madden NFL, programmers get sloppy when they are the only ones out there.