Which group is more suitable for metal nanoparticles functionalization?
Sulfur containing groups, such as thiols, are in general more strongly attached to silver nanoobjects than amine groups. However, the problem with sulfur and silver is that the strength and structure of sulfur-silver bonds depends critically on the local topology of the silver surface, and in-layer lateral interactions between stabiliser molecules can have a strong impact on this, so there isn't a clear pattern of reactivity of sulfur-containing groups and silver nanoobjects. You can easily get irregular, disimilar or patched coverage.

Amine is much more straightforward, links itself more weakly and is routinely used in the synthesis of silver nanoparticles as a stabiliser. However, if you want permanent functionalisation of the nanoparticle (vía covalent bonding), probably amine-silver bonding will be too labile.