Because of the disparity with the cost of diesel, some of the big petrols are really not that bad to run. In Scotland it was almost 17p/litre at one time, so , for the extra cost of the vehicle the supposed saving by running a diesel must have been eroded, particularly when its widely accepted that big 4x4's have fallen through the floor price wise.
You'll have to accept that a full sized job is required if you want to tow 3.5 tonnes though. A Daihatsu Fourtrak being one of the few exceptions, as its quite small, even in LWB, but could still haul 3.5 tonnes no problem. They are getting a bit long in the tooth now though. Most of the soft roaders will only tow 2.6-2.8 tonne, although some manufacturers offer a cost option to upgrade that to 3.5, for the current X5 for example, it was a £35 cost option when the vehicle was ordered, and could only be done at the factory. That is where the Discos and RRs win as they are all 3.5. tonne rated AFAIK
I couldnt fault my E53 X5 either (last model) and am very happy with my newer E70 X5, very relaxed cruiser and tows very well, I rarly go off road. The E53 4.6 was a stonker. The only complaint I have with the 4.8 is that the transmission shift can be a bit errratic, but only the 4.8 suffers this as the ecu tries to use the engine to brake a lot which means it holds ratios too long sometimes. Build quality is second to none in the E53's