They differ in speed and power, but their overall DPS and EPS is identical. Hidden Power has the advantage defensively, but Water Gun (due to its faster speed) can be advantageous for dodging.
Suicune doesn't get Water Gun however. The only Pokemon where this comparision can be accurately made is with Starmie. Trying to compare Suicune's Water Hidden Power to say the Water Gun of another Pokemon isn't fair, and several Water Gun users (namely Vaporeon) outclass Suicune due to base stat differences.
