Personas is more about who the user is, while scenarios are more about the situation the person is in. It sounds pretty simple to grasp, but when developing personas and scenarios the difference becomes hazy.
Persona contains a lot of information, including demographic, personal and social data, possible working environment description, his needs and goals. For example, persona's age could lead designer to some decisions in visual design of a system. Usage frequency could affect information architecture of a system.
Scenario includes persona and scenario not only aligned with persona's goals, but implements concrete user's task. So user goals from Persona divided on set of tasks, which are the core of every scenario.
Analysis of existing scenarios could give some insights and ideas on Person, so the Person could be improved, a kind of reverse engineering. But using scenarios as the only source for Person creation is bad idea. Scenarios are too narrow for this task.
To distinguish Person and Scenario you could use the sentence structure analogy: Person is a Subject, Scenario is a Verb. But distinguishing while brainstorming possibly not good idea. This could be done in analysis phase.
How do personas affect design decisions in a way that differs from scenarios?
You build persona's to help understand (and spread understanding) of who your users are. They're about breaking away from simply designing from use cases, scenarios and task analysis. They're written in a way that creates a richer image of what a user may do or feel when using your product. They help remind you that your product exists in a wider context that is different from one user to another.
Scenarios are built upon personas, but can personas be built upon scenarios?
Scenarios and personas should be part of a living and breathing design document, where each step and every discovery feeds back into (and improves) steps already taken. Thinking about scenarios with the personas in mind will inevitable provide new insight into your personas.
How can I differentiate between the two? Especially while I am brainstorming?
A persona makes the data more lively and addresses socio-political and ‘quality of life’ is- sues, including the values, fears and aspirations of the users. ‘They have names, likenesses, clothes, occupations, families, friends, pets, possessions and so forth. They have age, gender, ethnicity, educational achievement, and socioeconomic status. They have life stories, goals and taks. They are not ‘agents’ or ‘actors’ in a script, they are people.’ (Grudin and Pruitt, 2002). A persona must be liked.
If you're putting too much scenario into your persona in a way that doesn't add to the above, you're probably making them too specific to the problem you're trying to solve. Your persona's should be like people, not simply actors in a play about your product.
A persona is a rich picture of an imaginary person who represents your core user group. A scenario will have several of these personae covering different types of intended users and different roles. The personae will themselves be based on studies of actual users, observation, etc.