I am debating between the two. I am really worried about the older guests not being able to navigate a computer well...although really that is just a few people that come to mind. I am also worried that if I have an online RSVP people are more likely to put it off and forget. And traditional RSVPs are probably fun to get in the mail.
On the other hand, there is the whole added cost issue.
What is everyone doing? Have people found that the online RSVP works well?