The S197 platform is loosely based off Ford's DEW98 platform used in the Lincoln LS, and was originally designed to ride on the DEW Lite platform, however, production would have been more expensive. The DEW98 platform consists of 4 wheel independent double wishbone
suspension whereas the S197 platform consists of macpherson struts and a solid axle = less money to build. The Mustang is the only car that rides on the S197 platform, and it is technically "new" since 2005, because the DEW platform was modified so heavily that there are few carry-over parts.
The SN95 platform was first used in the Mustang in 1979, so yes it was 25 years old, and this platform was shared with over 10 different cars, but after 1992, the Mustang was the only car still used on this platform, and as we all know 2004 was the last year for the Fox platform.
Honestly, having owned both, the S197 rides and handles much better than the SN95. And the 2010 really didn't have any significant weight gain over the 2005-09 Mustang, so when the drivetrain and brakes are upgraded that will be where any extra weight comes from. The 370Z is a great vehicle, but it's more of a niche vehicle being that it only has 2 seats and doesn't have a price range from 22k to 48k. And being that the 370Z is only a 2 seater with a V6, I'd say even it is heavy for its size tipping the scales at over 3,200 lbs, but it's lighter and smaller therefore more nimble.