How Dummy Variables Should Be Modeled In A Linear Regression Model?
Since your task is to predict something, the better variable is the one that gives you a higher prediction accuracy. So you can simply test both and choose the one with which your model performs better.

However, I would suggest considering to engineer your own feature that incorporates information of both variables. For example, you could create three dummy variables: workday, weekend and holiday and include two of them into your model (to prevent falling into the dummy variable trap). Another option would be to only include the interaction terms between isWorkday and weekday.