Hi James,

The Phantom Flex has many new features not included in the Phantom HD Gold.

In regards to the max frame rates:

Phantom Flex :

1730 fps  (1440p)

2800 fps (1080p)

6100 fps (720p)

Phantom Gold

1052 fps (1080p)

1576 fps (720p)

Frame rates aren't the only consideration however. The HD Gold has a 2048x2048 sensor allowing for the use of Anamorphic lenses which the Flex's 2560x1600 doesn't allow. But in return the Flex has a native ISO of around 1000 vs 320 for the HD Gold. 

So while speed is important, it isn't the only consideration when picking between these two high speed demons.

Cheers!

Suny