Are you familiar with the Inverse Square Law? What sport(s) will you be shooting?
Light power changes as a square function relative to distance.
In other words if the subject distance from the light source increases 2x, the amount of light power that reaches the subject is 2x2 or 4 times less.
if the subject moves x closer to the light source, the amount of light power that reaches the subject is 2x2 or 4 times more.
So, you need more light for field sports than you do for indoor court sports because of differences in the distances involved.
For field sport use photographers add something like a Better Beamer to their flash unit.
The Flash X-Tender (Better Beamer) FX-3 Flash Output Booster for Canon 580EX, Canon 580 II and Metz 54-4
A further consideration is using a flash unit that has a zoom head that can change the shape of the beam of light the flash unit produces.
A wide beam is needed when the camera and light are close to the main subject, and a narrower beam is needed when the subject is further away.
Next you need to consider flash unit re-cycle time. Even the best flash units using an optimal battery type take at least a couple of seconds to re-charge after a full power flash.
One can work around that by using flashes of less than full power. But that also means less light, so you need a more powerful flash unit so you can turn down the power yet have the same amount of light as a full power flash from a less powerful unit.