I swear by the standard bottle jack that comes with the disco even with 265/75/16s mine reaches enough to change a tyre. Much safer and easier than a high lift jack.
But they are not as effective off road, plus they don't look half as cool when you bolt them to the back of your landy.
Bolting them to the outside of your Landy has an impact on how useful they are when you need them. One thing farm style jacks don't like is crud in the moving parts. Also to use one even remotely safely you need proper jacking points on your Landy eg. Heavy duty bumpers or rock sliders. Trying to jack from a standard bumper folds it in half.