The receptor binding domain(s) (RBD) of spike (S) proteins of SARS-CoV-1 and SARS-CoV-2 (severe acute respiratory syndrome coronavirus) undergoes closed to open transition to engage with host ACE2 receptors. In this study, using multi atomistic (equilibrium) and targeted (non-equilibrium) molecular dynamics simulations, we have compared energetics of RBD opening pathways in full-length (modeled from cryo-EM structures) S proteins of SARS-CoV-1 and SARS-CoV-2. Our data indicate that amino acid variations at the RBD interaction interface can culminate into distinct free energy landscapes of RBD opening in these S proteins. We further report that mutations in the S protein of SARS-CoV-2 variants of concern can reduce the protein-protein interaction affinity of RBD(s) with its neighboring domains and could favor its opening to access ACE2 receptors. The findings can also aid in predicting the impact of future mutations on the rate of S protein opening for rapid host receptor scanning.