What is the influence of weight on speed on a decline?

Nope. Galileo proved that in a vacuum, all objects fall at the same rate. http://www.grc.nasa.gov/WWW/K-12/airplane/ffall.html

In the real world on a bike, a heavier rider will be less affected by wind resistance, but this is a tiny effect. All sorts of other forces will make more of a difference, like friction, tire size, etc.

But in a “perfect” world, they’d go at the same speed.

To add to Ben’s point-if we’re talking about cyclists, then another issue is body mass density-i.e. how much fat/muscle does the cyclist have, and how that affects aerodynamics (fat presumably taking up more volume than muscle).

Here’s the physics and math for the inclined plane problem.

Caveat lector.