You are referring to a workflow in which a user must complete all steps versus a workflow in which a user does not need to complete all steps in order to complete a task.
In the first instance, having long forms or a high number of steps is not generally going to be user-friendly. The mitigating strategy is to provide a guided flow for the user (the steps in a 'wizard') or providing some sort of checklist (items to be completed in a long 'workflow').
So you should be looking at the nature of the content and any other types of constraints (business, technical, legal, design) and working out whether it is really necessarily to have complex processes. If it is not possible to change the process then you simply have to choose one of the two options ('wizard' or 'workflow' that best fit your requirements and implement a suitable strategy to help users with providing the right type of input.
I don't think it is a case of which one works best (you have identified that one provides more flexibility, but the trade-off is that it is more error-prone), but which one works best given your requirements. The question should be what is the best strategy to make these complex processes more user-friendly.