Over the course of the Discovery 3/4 and first generation Range Rover Sport's life, there were four variations on the Land Rover detachable tow bar. They all looked the same, but there were subtle developments. I had a tow bar dating from 2008, which on occasions would "rattle" a bit. It would be tight in the receiver when first inserted, but for some reason worked itself loose over the course of a journey. It was never in danger of dropping out as there's a huge pin that holds it in place, but I had the whole lot inspected by LR. It seems that there were no splines on the inside of the green locking handle on that model, which allowed it to turn just enough to allow it to drop by a gnats and "rattle". I bought a later model (and still have that) which remains nice and tight.
There were a few instances of detachable tow bars dropping out which gained publicity. Having seen the state of some of these though, it was clear that they'd been left in the receiver, and had never been removed. The locking mechanisms were corroded and obviously had never seen any lubrication or maintenance. They are meant to be removed and not left in permanently. LR define their use as "occasional" whatever that means? In my case, LR said that the detachable one suited my use (towing the caravan) perfectly. They're not really intended for 24/7 towing (their words, not mine).
If you do go for the detachable model (and there's no reason why you shouldn't), ensure that it is removed when you aren't towing, not only for the sake of maintenance, but they're also very easy to steal! Make sure that both the receiver and the bar itself is kept nice and clean and it has a bit of lubrication from time to time.
Finally, check that your car has been inspected. If so, it should have additional information in the handbook.