I'm assuming you are looking to evaluate a design/website, but cannot test it with end-users.
First, it will be helpful to define terms. A heuristic review is when you evaluate a design based on a set of general usability guidelines. A Journey Map is a document that articulates the steps a user takes to achieve their goals and the challenges/opportunities along the way. To create a journey map, you need to do some research, possibly talking to end-users or subject matter experts.
When evaluating a design, it's essential to know who the users are and what they are trying to accomplish. If you only focus on general heuristics, you may miss significant opportunities for improvement.
I recommend using a combination of both, often referred to as a cognitive walkthrough. Evaluate the site from the perspective of a user trying to accomplish a task to achieve a goal. Using the tasks and goals you identified in your journey map, try and complete the task in the design as if you were a user. See if there is anything that prevents the user from completing this task. Heuristics can identify problems to look out for and explain why certain things are a problem.