Con past and copy la meto aqui, por si algun alma caritativa quiere traducir, al menos los puntos mas importantes:
"We break down the comparison into six parts: Cars, Gameplay Modes, Driving, Graphics, Sound and Overall.
Cars
Wide selection of cars has been the meat of the Gran Tursimo series, and Forza has come with a formidable arsenal of its own, featuring over 200 cars. And Forza has more where it counts.
There's a niche draw to Gran Turismo, to be sure--diehard Skyline fans have long-yearned to drive the '71 GT-R that made the name plate famous in the first place... or the forgotten R31 Skyline, eclipsed by the sensation that was the R32 when it was released in 1989.
However, those couple hundred cars of obscurity seem overshadowed by Forza's refreshing influx of exotic cars. Finally, a 360 Challenge Stradale to grind around the curves without fears of $100,000 repair costs--something 10 variants of CRXs can't come close to compensate for. While GT4 does have some supercars such as the Pagani Zonda, it lacks the Italian powerhouses that are iconic to the word "sports car." Forza does have a selection of older cars, the notable including the 240Z, drifting icon AE86, and Toyota 2000GT.
On the flipside, GT4 seems more on the cutting edge with newer car models, featuring the new BMW 3-series before it was even released. Forza by contrast, has some models already getting phased out (such as 2004 Audi, which is already getting replaced by the redesigned '06).
On the customizing front, GT4 has a wider selection of rims... but this is far eclipsed by Forza's robust selection of exterior mods (side skirts, bumpers and hoods).
And nothing beats working towards driving a Enzo Ferrari.
Edge: Forza
Gameplay Modes
Gran Turismo starts with a major handicap for the simple fact that it has no online mode. Modes have largely remained unchanged, except for a race-management mode called B-Spec, and the scenic but dull Photo Travel. If you're unfortunate enough to not have a GT3 save handy, you'll need to go through the painful Driver's License courses--which although are easy to pass, have become more tedious than ever.
Microsoft's response to the B-Spec is the Drivatar, which features an adaptive AI that gets trained to how you drive. The DNA of the developer's driving styles can be seen in the AI cars themselves, which are driven by Drivatars tuned to their own habits.
Forza's online matches are brisk and addictive. It's a shock to make the transition if you're accustomed to exploiting AI flaws (running them off the curve, cutting them off when they try to pass)--because those human players will try the same tricks on you. Sometimes it's best to steer clear of the mess and wait for the opponent to wipe out--satisfying when the difficulty's turned up. Plus, all of your track times (even in offline career mode, provided you're connected to Xbox Live) get recorded, so you can work toward being the reigning track champion of Laguna Seca.
Edge: Forza
Driving
The PlayStation 2 analog controls has always been mushy, imprecise, prone to sudden jumps in sensitivity, and with a rubbery resistance that makes thumb motions needlessly more difficult. Contrast that with the Xbox controller, which has a nice even scale of sensitivity, and a resistance that feels natural.
Controller differences don't feel pronounced in driving games, but the slight edge goes to Forza, especially when using the right analog stick to accelerate/decelerate. The only downside to this configuration is the shoulder buttons are very sensitive, and you'll often inadvertently shift gears by touching them.
In terms of core driving and physics, both are pretty similar, but the obvious differences surface when taking account of Forza's damage modeling, which adds a new angle of game play. It's not accurate visually or physically (try to get even the bumper off of a Ferrari), but the approximation is a realistic deterrent to not plow into opposing cars and metal guard rails.
Edge: Forza
Graphics
Although Xbox hardware has shown itself to be more competent than the PlayStation 2, Forza has inconsistent levels of quality, especially when it comes to cars. According to Dan Greenawalt, Lead Game Designer, modeling was done by multiple development teams.
"All of our cars are about 15000 polygons (next to 10000 on PGR2 and 6000 on GT4)," says Greenawalt. "We used two external houses (one in Vietnam and one in India) to build 70% of our cars. Both of these houses did a tremendous job, but they had slightly different styles. The Vietnam team spent more polys in the interiors. If you look at the headlights and steering wheels as well as the fender lips you can start distinguishing which teams build which cars."
While the big-name makers and cars had a finish on par, if not better than GT4, lesser cars such as the R32 GT-R look worse than even Gran Turismo 3. In addition, the light reflections and shadows cast on cars are beautiful, but the sheet metal itself almost has a cling-wrap shine to it--less convincing than GT4.
The tracks themselves, however, are beautiful in Forza. The anti-aliasing and lighting contributes to a more life-like environment when compared to Gran Turismo 4, which aside from its improved reflections, still looks like GT3. And since you're staring at the road and scenery more than your ride, in the end Forza (narrowly) beats out GT4.
Edge: Forza
Sound
Having a compelling music track in racing games is important given the monotonous routine of racing--especially when facing the repetition of engine howls, screeching tires and scuffles. Unfortunately, the soundtrack is nothing short of electric-distortion cacophony. While there are a couple catchy sampled riffs, overall there's no change of pace in the music from the menus to the races--making the game music sound like one excruciatingly long track.
The grating music is partially redeemed by the high-quality sounds of tire traction, car collision and sampled engine sounds--which does have a small edge over Gran Turismo. Turn off or turn down the music and you'll still have a pleasant ride--but not as smooth as GT4
Edge: Gran Turismo 4
The verdict
Forza is by no means a perfect game--but the foibles are easily forgivable when you have a rich online mode, awesome cars, and satisfying exterior customization options. Gran Turismo 4 is still a noteworthy achievement, but with so few changes over the years it's starting to feel stale. Those who haven't been lured by Halo 2 now have one more reason to pick up an Xbox console.
Salu2.