We fabricated amorphous silicon n-i-p solar cells with two types of nanopatterned back reflectors using stencil lithography. One reflector type has a plasmonic grating that is embedded in the ZnO layer; the other one has a metallic grating patterned on top of the Ag layer. From comparing the short-circuit current densities of the two device types, we conclude that light trapping through grating coupling is more efficient than coupling of light through the excitation of localized surface plasmons. The back reflectors were patterned with dot arrays by evaporation of Ag through millimeter-size stencil membranes. The stencils themselves were patterned by wafer-scale nanosphere lithography. The dot arrays have a periodicity of 428 nm and efficiently scatter light in the near-infrared wavelength range. Both back reflectors types lead to the same morphology for the silicon films. This allows us a fair comparison of the two light coupling mechanisms. We found a 14% and 19% short-circuit current density enhancement for the plasmonic and for the metallic grating, respectively. The external quantum efficiency gains between 550 and 650 nm show similar guided modes resonances for both device types, but the excitation is stronger for the device with the metallic grating.