So...with almost 100K miles on my RRS, I can give you some decent advice - after running thru many trails in Moab (While rim trail), lots of the key feature trails in Colorado (Black Bear, Imogene, Mosquito, Halfmoon Creek, Ophir, Engineer et. etc.), and tons of other locations with not a single puncture on my Nittos.
If you stay with the stock tires in a 19 inch setup, there are only 2 choices - Goodyear MTR (very expensive and terrible in the rain on pavement) or Pirelli Scorpion (currently out of stock per those RRS owners hunting actively for this option).
If you have an RRS HSE model 2006-09, then your best bet if you plan on doing Moab/Colorado/Death Valley type expeditions is to get a set of 8x18's original LR rims - also available on line from time to time in either the rare 5 spoke pattern, or the more common LR3 10 spoke pattern. This will then allow you to fit 285/60/18 in either a Nitto Terra Grappler (2 options here, no real need to do the 10 ply in my opinion) or Cooper Zeon LTZ, both tires tested extensively by large groups of LR owners - both LR3 and RRS. Coopers tend be a slight bit stickier, wear out a bit faster and tend to get a bit noisier at the end of their life. I've been thru 2 sets of Nittos at almost 45k each set.
If you are planning on staying with your 20 inch rims, then also the Cooper Zeon LTZ - here you can run either 275/45/20 or 285/50/20...with just very slight rubbing under full compression in the rear, and very slightly rubbing on full steering wheel lock up front. The problem with this is zero room to air down, and from those running it...it's harsh on rocky trails compared to the 18's.
Hit me with any further questions...post up pics.
Also check out Rangerovers.net or RRSPORT.UK and you'll see a whole community of RRS owners - granted there are not too many of us who go offroad, but enough to give you guidance.
