We conducted a comparative study to investigate the efficacies of two different types of highly hydrophilic materials (i.e., silica nanoparticles (SiNPs) and zwitterionic polymers) for antifouling surface modification of polyamide thin-film composite (TFC) membranes. Dense layers of SiNPs and zwitterionic polymer brushes were grafted on the membrane surfaces via dip-coating with aminosilane-functionalized SiNPs (i.e., SiNP-TFC membrane) and surface-initiated atom-transfer radical-polymerization of sulfobetaine methacrylate (i.e., PSBMA-TFC membrane), respectively. With the same degree of enhancement of surface hydrophilicity and identical surface roughness, the PSBMA-TFC membrane exhibited significantly higher fouling resistance than the SiNP-TFC membrane in adsorption tests of proteins and bacteria as well as in forward osmosis (FO) dynamic fouling experiments using alginate as a model organic foulant. Chemical force microscopy measurements revealed that membrane-foulant electrostatic attraction aggravates organic fouling of the SiNP-TFC membrane to a certain degree, but the primary fouling mechanism is the complexation of organic foulants with carboxylic groups on the polyamide membrane surface. We attribute the lower fouling resistance of the SiNP-TFC membrane to the high density of surface carboxylic groups that may still be accessible to foulants as well as to membrane-foulant electrostatic interaction. On the contrary, the zwitterionic polymer brushes effectively shield the surface carboxylic groups and provide steric hindrance against foulant adsorption due to significant hydration of the zwitterionic brushes.