Determine which molecule has stronger intermolecular force?
Look at the boiling points.  Interaction is based on geometry (surface area) and polarizability, among other things - and the phase: solid, liquid, vapor.  Molecular weight is not diagnostic.  SiH4 is "fluffy" versus methane being "hard."  Compare the melting and boiling points of decane and isodecane  versus adamantane, all being $\ce{C_{10}}$.  Compare the melting and boiling points of n-octane versus 2,2,3,3-tetramethylbutane, both $\ce{C8H_{18}}$.  Complicated.

You might check your MW for methane.  Bells and whistles should have gone off, $\ce{CH4}$ versus $\ce{SiH4}$.