I am going through the wedding invitation stage myself. Most people send out 'save the date' cards months in advance then the invites a couple of months before the wedding.
We actually emailed everyone the date and destination, so they were aware of our wedding and we had an idea of who will and who wouldn't come. They could also book time off work if need be.
Send out an invitation which is a hard copy for guests to hopefully put on their pin board. They should also send you back the rsvp section of the invite.
I think doing traditional invitations is nice and formal, but email people as well.