We present research into the capacity and spectral efficiency of various spatial and mode division multiplexed transmission schemes in multimode optical fibres, including MIMO schemes based linearly polarized (LP) modes and orbital angular momentum (OAM) modes. We estimate their spectral efficiency by calculating the transmission matrix and considering mode coupling under different schemes. We also consider the signal-processing prefix for LP-MIMO and OAM based technologies to calculate the effective spectral efficiency. Simulation results show LP-MIMO using step-index fibre can support higher spectral efficiency than both LP-MIMO with graded-index fibre and OAM, while LP-MIMO with gradedindex fibre has similar levels of spectral efficiency as OAM. We further compare the computational resources required to support the signal processing algorithms in order to achieve certain amount of capacity or spectral efficiency and reveal the scaling of computational resources with capacity in different MDM schemes. Considering the implementation of MIMO algorithms and any processing required to support spatial light modulator (SLM) based optical crosstalk equalization schemes, we demonstrate that OAM-based MDM has the lowest computational resource requirement and offers a much higher spectral efficiency with limited computational resource in both processing power and memory size requirements resulting from intermodal group velocity dispersion in multimode fibres. We also show that OAM based schemes could save significant signal processing energy overhead due to its low computational resource requirement.