The usability of smartphone software presents unique challenges as compared to desktop software. Both aesthetics and functionality play an important role in mobile interface design. In this paper, we examined the usability of the iOS Touch ID feature with different user prompts. We compared three different types of user prompt designs for the touch ID feature, including a user prompt with no guidance (NG), a user prompt with aesthetic-first guidance design (AF), and a user prompt with functionality-first guidance design (FF). An experiment with 30 participants showed an improvement for 90% of them when using the FF prompt for the fingerprint inputs, as compared to when using the AF prompt. Additionally, the fingerprint inputs were improved for all participants using the FF prompt as compared to the NG prompt. We concluded that user prompt designs do have a material impact on the usability of mobile software, and that functionality rather than aesthetics should be the primary consideration in user prompt design.