To most folks they are interchangeable. Catnip and catmint are closely related perennial herbs in the mint family. If I had to state their greatest deference it would be that catmint has more ornamental value. Catnip is more of a medicinal herb.
Catnip (Nepeta cataria) and catmint (other Nepeta species distinctly different from N. cararia) both thrive in full sun and both can tolerate significant drought once established. Neither tolerates acid soil particularly well, and both prefer well-drained conditions. In our area, long periods of hot humid days can be problematic.
Catmint varieties normally grow 12-18 inches tall, while catnip might reach heights of more than four feet. Catnip also tolerates shade better. Both are very winter hardy.
These mints have tooth edged leaves that emerge in groups of two. Stems are square, and both have sprawling growth habits. Both are easily propagated by seed or stem cuttings. Seeds are very small and produced in large numbers.
Catmint also blooms more profusely. During summer it can be a sea of purple. Catnip has far more foliage, is more stemmy and sports short spikes of lavender flowers.
These herbs attract hummingbirds, bees and other pollinators. Deer avoid them. In fact, some people plant catnip and catmint simply to repel deer. However, in areas with dense cat populations it’s sometimes difficult to get these perennial herbs established. Cats will roll in them and root them out.
These pungent herbs belong to the genus Nepeta and contain the compound nepetalactone. This is what drives cats crazy. The chemical is somewhat stronger in catnip than catmint.
Though people have told me differently, from everything I’ve read, no corroborating evidence exists that catnip has lasting long-term effects on cats. However, nepetalactone will alter their personality in the short term. Many cats aren’t affected that way, and some show no interest in it at all.
Some research claims catnip and catmint to be effective insect repellants. However, their effectiveness is short-lived. Constant re-applications are necessary. I’ve heard these herbs can be planted around barns to keep rats and mice away. I suspect that’s mostly because the plants would draw cats.
As far as humans go, both these herbs are commonly used. Catmint makes a relaxing herbal tea. Most folks might want to sweeten it. Both mints are used in cooking, but flavor can be overpowering in large amounts.
Leaves can also be eaten raw in salads. It’s my opinion that the smell of catnip is better than the taste. In concentrated doses it can be used to promote vomiting. If that doesn’t sound appetizing, sometimes catnip preparations are applied directly to the skin to relieve pain.
Both catnip and catmint are used as diuretics. They increase urine flow. Lithium is used to treat bipolar disorder, depression, schizophrenia and some eating disorders. Catmint or catnip use can interact and limit how the body rids itself of excess lithium. That’s not good.
Catnip is often used as a sedative to cause drowsiness. This could pose a problem for people already taking medication for this. It’s always critical to consult your medical professional before embarking on any new herbal medication regimen.
Ted Manzer teaches agriculture at Northeastern High School (tmanzer@ecpps.k12.nc.us).
tedmanzer 
They really are two very distinct personalities. I have never noticed that cats bother catmint, even where catnip must be grown in hanging pots to keep them out of reach from bobcats.
There’s not much of a wildcat problem here, but feral cats tear both of them up. True, catnip more than catmint, but they do roll around in the flowerbeds, and catnip is usually too wild-looking to mix with most perennials.