I called Speedotron just a few minutes ago, and tried to compare my 2401SX versus the current 2401SX2. And they didn't give me much difference since they thought I had a different pack like a 2403 or something.. But the SX lines are indicated to be much more robust having better resistors and matched capacitors. And I metered the flash durations to be near 1/900 at full power. So I can get fairly crisp action sport flashes all day long.