The vibrational excitation processes which occur in molecular beam experiments on H-2 molecules, and using H+ or H- as projectiles, are discussed from the theoretical viewpoint of the microscopic quantum dynamics and in relation to the various features of the two potential energy surfaces. The present study employs the vibrational close-coupling-rotational infinite-order sudden (VCC-RIOS) decoupling scheme and analyzes in detail the differences of behavior of the various inelastic differential cross sections in the small-angle region. It is clearly found that two separate mechanisms can be invoked in the two systems to explain the differences in efficiency between the two excitation processes. Such mechanisms can be related in turn to specific features of the two potential energy surfaces and to their bearing on the final dynamical observables. Rather good agreement between calculated and observed cross sections is found for both systems.