While the off-road part is true, the rest isn't. The Tahoe has more room and the road manners are night and day better than the Disco. It's fuel efficiency is better than the Disco as well if you want to get technical (within a mpg or two according to EPA ratings.) Those I know who've had Discos (and my previous one) always got worse than the EPA rating by a good amount. The people I know with that generation Tahoe have got the stated amount or slightly better usually. Of course there's too many factors to judge this as any "fact."
Have you really driven an example of each? I can't believe anybody would come away with this conclusion.
I have an 04 D2, and my father has a 99 Yukon. The difference between them is staggering.
I'll give the Chev marks for fuel economy, and cargo space. No doubt it beats the D2 handily in both. It gets 18-19mpg on the hwy, and I've gotten 15mpg towing a car with it. The D2 gets about 16 on the hwy, 13-14 towing. The big V8 in the Chev also feels more relaxed while towing, kicks down less, etc. The back cargo area is about twice the size of the Disco, and there's more width in the back seat for 3 adults.
But other than that... Road manners? You're kidding! First of all, the seats are amongst the worst things ever foisted on the unsuspecting public. No support, soft, ill-formed, bottom cushion too short... I can barely stand to sit in them. The interior of the 99's is the last of the worst of the 90's era GM corporate interiors. Horrible buttons and controls, and an absolutely "style free zone". The brakes are absolutely horrible, right from day one. Overboosted, but with little actually stopping power, long travel, terrible ABS system. There's zero steering feel, and the suspension is typical 90's GM oversprung and underdamped, the only thing stopping it from oscillating forever is all the friction built in. I've been in the back of the truck when towing a 2500lb pop-up trailer, and even after a short distance it had a terrible pitch oscillation (thing motorboat in choppy water) that threatened to make me sick. Lastly, the headlights are just terrible. Small inefficient reflectors using the worst bulbs available in the industry. The only hope is with some auxilliary lights, which won't help you when there's oncoming traffic.
By comparison, the D2 had fantastic european style supportive seating. The interior layout, while sometimes quirky, at least it has some euro-technical character and outdoorsyness with the map pockets on the headliner, etc. The quality of all the materials, the carpet, leather and upholstery, headliner, etc, all better than the GM. The seating is more upright for both the front and back passengers, more comfortable as long as they don't have really long legs, and the stadium seating is nice. The braking and steering is better than many cars I've driven. The suspension is well tuned with no chop or wallowing, without being harsh. Well, there is a bit of harshness on sharp edged bumps due to the SFA and front suspension design.
Realiability... the only thing I'll give the GM is that parts are cheap and available. But if I had to go a long way, I'd take the Land Rover. Over roughly the same number of miles, the GM has had way more problems that my LR and that despite the fact the GM has led a mostly on-road life, and mine has been used offroad. Some of it stemming from from the weird CSPI fuel injection system, and some from the ignition components. Both caused hard to diagnose intermittent misfires. The GM has had more leaks, trouble with the TC case, brakes... I don't even know all of it.
We've driven around on some dirt roads together as well, and the Disco can handle corrugated roads much better than the Yukon. I'd never even think of serious off-roading in the GM.
Bottom line, you litterally couldn't give me a GMT400. For a long time, I was completely sworn off of trucks. Things like this, the U150 Explorer and P150 Ranger, etc I did everything I could to keep my trackday car streetable, and did everything I could to keep my other tows light and towable by cars. It wasn't until I drove the Land Rover that I discovered I could actually enjoy a truck.