Lookup or Formula Field, which to use?

Lookup or Formula Field, which to use?

I have a field in called Primary Business in the Accounts Module and I want the contents of that field to populate in a field in the Contacts Module called Accounts Primary Business. My preference is that it populates right away. 

I've tried both lookup and formula fields, and can't seem to get it to work.

Will this be searchable in contact reports and populated for report exports?

I have the enterprise version. Thanks for the help.