Removable every time. It’s not just your own forgetfulness that you have to accommodate for: I’ve come back to a car more than once to find the person behind me has become hitched on to my fixed bar while they were parking.
Witter, bosal, westfalia: they’re all very similar mechanically. Just beware that modern vehicles will inevitably require coding to the electrics and non-approved parts, even when fitted by the dealer, can have the effect of invalidating your entire electrical manufacturer’s warranty which may require the replacement of the full wiring loom (thank you very much MSB Skoda in Darlington, you bunch of incompetant w@114R$!).