The differences between the RF-/DYNAM Pro add-on modules Forced Vibrations and Equivalent Loads are explained in FAQ 002062 .
When the settings are the same, the same results should be calculated in both add-on modules. However, this does not apply to existing nonlinearities because the RF‑/DYNAM Pro add-on module generally does not consider nonlinearities. When displaying the results in the Forced Vibrations add-on module, all nonlinearities are thus ignored. In contrast to this, the equivalent loads are calculated on a linear system and the exported load cases are then applied to the real structure in RFEM 5 or RSTAB 8 - that is, with all nonlinearities. This may lead to inconsistent results. If you deactivate the nonlinearities for the exported load cases, you should obtain the same results.
The method of considering nonlinearities in the response spectrum analysis is described on the basis of tension members in FAQ 002237.