Differences between PBI RS and PBI service

Hi,

I'm working on a PBI Project on a Bank, and they are still to decide if they are going to PBI service or RS. Though this is a decision that is above my pay-grade, i would like to help on the decision. 

I think that in the end we'll have PBI and Reporting Services (paginated, and other stuff) available for the users, and I'd like to ask:

1 - what are the differences between going PBI service or Reporting Services, and other implications?

2 - is PBI RS so quickly updated (and monthly) as the online service?

3 - what can of security measures does PBI service implement?

Thank you,

Best Regards