They are completely different substances, with completely different molecular structures:
Methyl nicotinate is a very strong vasodilating agent and can be in some cases highly irritating to the skin even at very low concentrations (0.1% or less). Menthyl nicotinate (Nicomenthyl) effectively enhances skin microcirculation without causing bothersome hyperemia or irritations, on the contrary it generates a pleasant hot-cold sensation in the area of application.