In America, most offroad situations are usually defined by the Rubicon Trail, hence the reasons why Jeep sells the "Rubicon" model of the Wrangler, and that most Jeeps are tested their to get their "Trail Rated" seal of approval.
The Wrangler is certainly a best bet in almost any situation, be it sand, mud, snow, etc while it would seem the Defender would do best in the mud and other "British" off-road situations... I think of the Top Gear test of the LR3/Disco in that situation, and I'm not sure why.
Either way, they are argueably the two best off-roaders available for civilian use these days, especially once with a "reasonable" price as compared to the VW triplets, which aren't too bad off-road either.