Hi Eric,
Maybe I can help, I've used both the OverLand and OverCamp, they are very different tents. They are similar, both being made by AutoHome (Zifer) in Italy.
First, putting the rooftent on a trailer or vehicle. A lot depends on the trailer. If your trailer has shock absorbers, then it is OK. Many offroad trailers have really basic suspension that is brutal to anything in or on the trailer when you are on rough roads.
The most obvious difference between the tents is that the OverCamp opens like a book while the OverLand opens like a wallet. If you are carrying things like kayaks, boards, bicycles or hang gliders, the OverCamp is the obvious choice, designed to share the load bars.
The OverLand is designed to be lighter with extensive use of alloys, where the OverCamp is designed to be more basic with plated steel for supports, so the OverLand is lighter overall. Even though the fabrics are the same the OverLand compensates by using a thicker weave. The drawback is the fabric is really expensive, I asked for some for a boat cover and was horrified by the price and went with a sunbrella acrylic, not nearly as good. The fabric is a polyester acrylic and cotton blend and seems to have the best of both, the cotton swells making the fabric water repellant, but still breathes. The polyester helps to wick water so the fabric is rot resistant and holds up well in the sun.
The OverLand is much more sophisticated, being made since the 50's. The OverCamp is newer, more basic and heavy. I think they are both equally rugged and durable.