Hello Mikey
Both the Sorrento and the Merc are both rated to tow 3.5T which is well beyond the 2T that Philip wishes to tow.
The only time that the difference in these two car might be for Philip, is if he had only passed his driving test after Jan 1997, when, the standards test only allows the driver to tow a trailer up to 100% of the kerb weight, but he would also be scuppered on the train weight which would exceed the categories limit of 3500kg
Otherwise legally there is no problem with either car.
Technically it is sensible to keep the ratio as favourable as possible. And of course there is no guarantee that combination will tow well, that depends on a whole host of factors, one of which is weight ratio, but loading, tyres, suspensions and not least driving style will all affect the towing experience. - but that also applies to every other combination.
Philip I cannot offer any suggestions on which of the two vehicles to choose as I have never driven either model. But it is easy to become blinkered and it might be worth considering a wider range of vehicles.
Both the Kia and Merc are considered to be large 4x4's, and whilst they area capable towers, All 4x4's are mechanically more complex, and have the potential to be more expensive to service and repair, and with the weight and the friction the extra mechanics impose they will use more fuel than an equivalent two wheel drive model. How often have you honestly come across a towing situation that only a 4x4 could master?
Even the most ardent caravaners spend most of their driving without a caravan in tow. So unless you are mud plugging everyday, it might be worth looking at what else can tow your caravan, and might also provide a better day to day driving experience.
You mention the Audi, clearly you are not jumping to the conclusion that large 4x4's are the only choice, in fact is a 4x4 actually necessary? There are other large saloons and estates that are technically capable of towing 2T.
I personally use a 2.2l diesel Renault Espace to tow a 2T twin axle trailer, and it has always been a pleasure to drive, offering the high driving position similar to a 4x4, the flexible space for passengers and luggage, and good economy. More recent models offer a 3L diesel with auto box. There are of course a choice of MPV's - both Kia and Merc offer models. - it might be worth looking.