What to use in CRM Online? Business rule, Workflow, Javascript?

Hello,

I need some advice for CRM online regarding what is the best method to include functionalities.

Example: In Opportunity, I need to set "A" field value to "B" field. I understand, I can achieve this using Business rule, workflow and javascript.

Easy way I can think of is by using workflow i which I can set conditions like Record is created etc.

Will creating n number of workflows hamper the system in any way in regards to speed?

For small or big changes, is workflow preferable or javascript?

Any help would be appreciated.

Thank you.