It's just right there at the beginning of each video. The tail lights on the GTS car are much more defined and polished than in Forza, the lighting is much better in GT... overall, it's hard to deny IMO that the GTS example is far better-looking. Strangely, the Forza video linked by
@emula doesn't include an interior view. However, if you compare the interiors as shown in the video below, the difference is extremely noticeable. The GTS version's interior is less polygonal and more polished by a country mile. Look to the left of the driver's hand. There are some very noticeable polygons above the air vent in the FM7 version, while it's smooth in the GTS version.
I mean, I'm a fan of both franchises and have poured hundreds and hundreds of hours into each, but the graphical difference is night and day here. That said, FM7 can look drop-dead gorgeous in many circumstances. Ever raced at midnight at the Nurb with the HUD off and just basking in the glow of the gagues? Not even being sarcastic, it's simultaneously a tranquil and terrifying experience, and it frankly beats "night" racing at the Nurb in GTS.