Plan for when the work is new, unknown and unique. Patterns are for common problems.
Some patterns and plans are also presented as processes. I'll leave the idea of work as a process to you and the comments below.
If you know the work you'll need to do well and intimately ahead of time, it's a pattern. We find patterns all over the place in project management, especially in strongly process oriented organisations. Here are a few examples;
- User acceptance testing pattern; You set up a replica of your production environment, write up test cases around the system capabilities, execute the tests an report on the outcomes. All good? Go live.
- COTS Product evaluations pattern; Define your needs, investigate options, compare, select.
- Scrum pattern; Plan a sprint, execute, demonstrate the outcomes, reflect. Repeat.
Scrum for example is a pattern. The details within the sprint need to be planned (hence it kicks off with a planning session.) Some of those details may also be patterns. For example, in my team the QA process is a pattern within the sprint that is repeated several times.
So, if you know the activities you'll need to do, it's a pattern. Patterns save time and bring consistency.
So, as I said above, that leaves plans for the uncertain, untried and unique work. Something implicit in this statement is that plans have to accomodate uncertainty, and allow for details to emerge as you travel with them into the future.
Plans are important because they help you think ahead though the uncertainty and complexity. Plan's don't have to get you to the right answer every time. Their job is mainly to help you anticipate some of that uncertainty.
Projects, at least the ones I have seen, use both patterns and plans to organise the work.
Both planning and patterns require something to make them work; good judgement.
When should you use a plan and when should you follow a pattern? When do you customise a pattern and when do you populate plans with patterns? You need to think about these things, and that part of the organising process is planning.