Lipid nanoparticles (LNPs) are of significant interest as delivery systems for various RNA therapeutics, not least due to their outstanding success in applications including the COVID-19 vaccines and the siRNA therapeutic Onpattro®. As LNPs consist of different lipids, the lipid composition determines key properties of these particles. This study examines how lipid composition, especially helper and PEG-lipids, and RNA cargo (siRNA and mRNA) affect LNP performance in pulmonary delivery. By comparting two different helper and two different PEG-lipids, we assessed the impact on fusogenicity and endosomal escape, in vitro transfection efficiency, and subsequently protein corona formation. Their in vitro performance was assessed in the air-liquid interface (ALI) cell culture model, a sophisticated in vitro model of the lungs. Our results demonstrated that transfection efficiency and stability differ between the helper lipids DOPE and DSPC, depending on the RNA cargo. These differences can be attributed to the structural differences of the lipids and the different properties of the RNA molecules. Our investigations further demonstrated successful mucus penetration of all LNPs and 24-42% gene silencing in vitro. We also explored mucus proteins/LNP interactions in human lung mucus, finding distinct protein corona formation for DSPC- and DOPE-containing LNPs. This comprehensive analysis highlights the critical role of helper lipids in combination with RNA cargo in determining LNP properties, efficiency, and in vitro performance, providing valuable insights for optimizing RNA delivery systems.