There has been much discussion in the literature regarding the determinants of ribosomal decoding speed during the mRNA translation process. Computational results based on mechanistic models for ribosomal kinetics have predicted a major role for near-cognate and non-cognate tRNA binding competitive behavior on translation elongation rate. In contrast, a recent experimental study proposed the difference between Watson-Crick and non-Watson-Crick type of tRNA decoding as the main determinant of translation speed. We model stochastically the translation process using a mechanistic model for ribosomal kinetics that accounts for both competitive tRNA behavior and Watson-Crick and non-Watson-Crick types of decoding. With this framework we identify the relative roles that tRNA binding competition and decoding interaction type play on translation elongation rate, and we explore the conditions that allow for optimal protein synthesis.