My discovery has 16 inch rims however my dads (who i live with) has 18". At the time he purchased his tyres there was difficulty in obtaining any non road biased tyres in that size with the correct speed rating however General grabbers do the AT2 which are an aggresive looking all terrain. On road they are very very good and in the wet aswell, so good i bought a set for mine!
They wear well drive lioke a normal tyre (they perform better than my brigestones which had 6mm of teread left) in the wet and dry, and landrover experience centres are often using them!
We ha#ve bouth 10 AT2 between us and have no compaints but lots of positives.
I cant comment on MT's as i have never owned any!