OpenTable and ResDiary are both popular reservation management systems for restaurants. However, there are some key differences between the two:

1. Features: OpenTable provides a comprehensive reservation management system that integrates with a restaurant's point-of-sale system, allowing them to manage their reservations, guest data, and seating charts. ResDiary offers similar features, but also provides additional tools for marketing and loyalty, as well as a more robust guest data management system.

2. User interface: OpenTable is known for its user-friendly and straightforward interface, making it easy for diners to search for and make reservations at restaurants. ResDiary has a more complex interface, but provides a more comprehensive set of tools for managing guest data and marketing.

3. Pricing: OpenTable charges a per-cover fee for reservations made through its platform, while ResDiary charges a monthly subscription fee.

4. Integration: OpenTable integrates with a wide range of point-of-sale systems and other tools, making it easy to manage reservations and guest data. ResDiary also integrates with a number of tools, but may have fewer integrations compared to OpenTable.

Ultimately, the choice between OpenTable and ResDiary will depend on the specific needs of your business. If you're looking for a simple and straightforward solution for managing restaurant reservations, OpenTable may be the best option. However, if you're looking for a more comprehensive solution for managing your hospitality business, ResDiary may be a better choice.