RSVPs are needed for all weddings, however, you get the choice between mail-in and electronic.
Personally, I never suggest mail-in, I love a good paper invite (and so will your guests). However, safe your self some trouble and use a wedding website for RSVPs.
Here's why mail-in is more stressful than it's worth;
First off, your mailbox is going to fill us and things are going to get lost.
Second, this system requires a whole extra system that you need to manually input their RSVP into, when your guests could just be doing that themselves.
Lastly, it's old school – most people don't want to mail things back, think about the last time you were asked to mail something and how long you procrastinated before doing it.