I voted NIC, but I've actually got a NIC/x-pen combo because free-roaming isn't an option at the moment and my house is too open to baby-gate them into a room - they've got about 70 square feet of run space plus a 3.5 story condo (there's a little landing between the second and third floors and it's one of their favorite sleeping spots).
if I had to pick one or the other (or if free-roaming was an option), I'd say I prefer NIC just because my bunnies (even the one that's not a big jumper) absolutely love having upper levels. Gaz had to be taught to use the levels, but she likes to follow/copy Nala so she spends as much time up high as Nala does.