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?
From Sleeswijk Visser, F (2009) Bringing the everyday life of people into design :
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.