Geometric factors of nanofillers considerably govern the properties of conductive polymer composites (CPCs). This study provides insights into how geometrical alteration through nanotube-to-nanoribbon conversion affects the electrical properties of solid and microcellular CPCs. In this regard, polyvinylidene fluoride (PVDF)-based nanocomposites are synthesized using both the parent multi-walled carbon nanotube (MWCNT) and its chemically unzipped product, i.e., graphene nanoribbons (GNRs). Theoretical and experimental results show that GNR-based composites exhibit 1-4 orders greater conductivities than MWCNT-based composites at the same filler loading because of the larger number of filler-filler junctions as well as the significantly greater contact areas. On the other hand, the conductivities of MWCNT-based and GNR-based composites are significantly increased by 230 times and 121 times, respectively, through microcellular foaming. The effective rearrangements of rigid MWCNTs and flexible GNRs (having 4 and 5 orders less bending stiffness) for network formation during cellular growth are compared. The GNR-based composites also exhibit a superior dielectric permittivity (e.g., 2.6 times larger real permittivity at a representative frequency of 103 Hz and a nanofiller loading of 4.2 vol%) compared to their MWCNT-based counterparts. This study demonstrates how the modification of the carbon fillers and the polymer matrix can dramatically enhance EMI shielding.