1. Pricing Model: Day Optimizer offers a subscription-based pricing model, while Invoice Ninja allows users to choose between a self-hosted version with one-time payment or a cloud-hosted version with a monthly subscription fee.
2. Focus: Day Optimizer focuses on managing and optimizing daily tasks and schedules, while Invoice Ninja is primarily designed for invoicing and billing purposes.
3. Features: Day Optimizer includes features such as time tracking, to-do lists, and calendar integration for efficient task management, whereas Invoice Ninja provides a wide range of invoicing features like customizable invoice templates, recurring invoices, and expense tracking.
4. User Interface: Day Optimizer has a clean and minimalist user interface designed for quick access to daily tasks, while Invoice Ninja offers a more comprehensive dashboard with detailed financial insights and reporting tools.
5. Integration: Day Optimizer might have limited integration options with other software and platforms, whereas Invoice Ninja offers integration with a wide range of third-party applications like payment gateways, CRMs, and project management tools.
6. Customer Support: Day Optimizer may have limited customer support options compared to Invoice Ninja, which provides extensive customer support through various channels including live chat, email, and a knowledge base.

In Summary, Day Optimizer is focused on daily task optimization with a subscription-based model, while Invoice Ninja is oriented towards invoicing with flexible pricing options, diverse features, a comprehensive user interface, extensive integrations, and robust customer support.