To combat the global shortage of mental health services, new solutions - such as computerized therapy options - have to be found. While research in this field has been ongoing for several decades and approaches such as chatbot-based cognitive behavioral therapy (CBT) have already shown to be effective in reducing symptoms of depression for younger adults, voice assistant-based (VA-based) approaches have thus far not been investigated deeply. However, especially for elderly people with depression VA-based systems could yield benefits - such as the avoidance of physical accessibility issues. Therefore, we propose the design of a VA-based system capable of delivering selected methods from CBT to elderly users with depression in order to investigate its usability. To assess the usability of the conceptualized system in comparison to a chatbot-based approach we conducted a randomized controlled A/B testing experiment with 14 participants. Results indicate a good usability and acceptance of the designed system and a preference for the delivery of CBT-methods via voice assistant rather than via chatbot, especially among elderly participants.