Yes, separate: rails for the app and statically generated site for marketing. It's generally a good practice (easier to change one without affecting the other, easier to give access to repos separately etc.)

But ultimately your decision should be based on development speed rather than cost when starting out. If you can ship faster with a single rails app, then do that. If you ever get to the point where you get so much traffic that cost becomes a concern, it's a good problem to have and easy to solve.