Virtual keyboards of different smartphone platforms seem quite similar at first glance, but the transformation from a physical to a virtual keyboard on a small-scale display results in user experience variations that cause significant differences in usability as well as shoulder surfing susceptibility, i.e., the risk of a bystander observing what is being typed. In our work, we investigate the impact of both aspects on the security of text-based password entry on mobile devices. In a between subjects study with 80 participants, we analyzed usability and shoulder surfing susceptibility of password entry on different mobile platforms (iOS, Android, Windows Phone, Symbian, MeeGo). Our results show significant differences in the usability of password entry (required password entry time, typing accuracy) and susceptibility to shoulder surfing. Our results provide insights for security-aware design of on-screen keyboards and for password composition strategies tailored to entry on smartphones.