We consider twin-peak quasi-periodic oscillations (QPOs) observed in the accreting low-mass neutron star (NS) binaries and explore restrictions to NS properties that are implied by various QPO models. For each model and each source, the consideration results in a specific relation between the NS mass M and the angular-momentum j rather than in their single preferred combination. Furthermore, the inferred restrictions on NS properties (or QPO models) are weaker for the low-frequency sources than for thehigh-frequency sources.