I think there are benefits and drawbacks to different gravity to earth. As you say, less atmosphere would help with air resistance, but they'd be in contact with the ground less often too and therefore have less opportunity to give themself forward momentum.

We also have to think about breathability of the atmosphere. If they need to wear some gear in order to breathe, that will slow them down

On balance, I think the boring answer of earth seems like the correct one

Top performing sprinters accelerate faster that 1g, so slightly increasing the gravity would probably make them faster on very short distances.
For long distances it would probably be some low gravity jumping limited by the relative speed between foot and torso that you can create before touching the ground and decelerate before overextending the leg.