Which observation method is harder to utilize for nailing down the mass of an Earth-sized exoplanet?

The gravitational influence exerted on its star (the radial velocity method)

For smaller exoplanets, specifically those categorized as Earth-sized, confirming their true composition requires accurately determining both their size (radius via transit) and their mass. While detecting the slight dimming of starlight during a transit provides a relatively precise measurement of the planet's radius, determining its mass through the radial velocity method—measuring the slight wobble or gravitational tug the planet exerts on its host star—is significantly more challenging. This radial velocity signal is much weaker for smaller bodies, making accurate mass confirmation a much more sensitive and difficult task requiring advanced follow-up observations.