I'm not sure why to be honest but I'd suspect it's based on the engine outputs.
One key thing to think is do you want one with 5,6 or 7 seats.
Tech packs are very useful
Check the mmi on the vehicle. As higher the better.
Check service history. Mainly for proof of gearbox oil services and if it's had any work done the air suspension as that's the biggest problem with them.