If the choice is between a disco and a sport then your repair and maintenance costs are both governed by Land Rover (parts) and whomever does your repairs. If you plan on doing more technical off road driving then you must expect you will start breaking things as you try harder stuff. It's part of the sport.
If you enjoy getting unstuck and doing field repairs then extreme off roading is perfect. At that point most people build a trail rig and tow it to the trailhead. This opens up the possibilities of modifying your downer vehicle in ways that would make it unsafe or uncomfortable on the highway. Also when you do munch it good you can simple drag it back to your trailer and tow it back home.
If instead you prefer to explore wherever you can safely go the Rover is really the perfect vehicle because it has enough off road capabilities to get you most places while at the same time being comfortable on the highway for long distance driving.
I have a good friend who had I convinced to buy a P38 just like I had. We both did some wheeling then he got frustrated with the repair costs and bought a Jeep Rubicon. He then had it built up with a lift, gears, bumpers, rack. etc. We took a long trip out to Moab. When we got there we did some trails but he didn't want to really push the rig because it was his daily driver. After that trip we both agreed that the truck was a bit small for 3 people on a long trip and he bought a BMW X5 which is a perfect daily driver.
Me on the other hand I just bought a RRS and am now in the process of adding off road parts so I can go wheeling again.

I plan on doing some technical trails but nothing too extreme where I could total the truck. At some point I would really like to have a short wheelbase diesel off road truck that is dedicated just for off roading. Like you I'm waiting for the release of the new Defender Diesel.