Hi Amanda -
Good question, we have done both. I would ask your question with a question - what are you planning to do with these Contacts? Are you first going to nurture them with emails to warm them up to pass them over to Sales? Or is Sales chomping at the bit to call them right away? If it's the first option I would import into Marketo first and then pass them over to SFDC when they reach your definition of a lead threshold (read scoring). If it's the second option I would have them imported into SFDC first to have Sales contact them and you can then run campaigns in parallel if you want. I hope that helps - let me know if you want to discuss further.