Answer to the question is it depends what you want to use it for. As you say you'll keep one of your 100s, it's not like you'll need the Touareg for "off-road" stuff.
My old man has the V10 toerag and uses it for "overlanding" around Southern Africa and I have done some fairly rough stuff through Lesotho in it with him. The Touareg is a very different vehicle to the 100/200 - much more car-like and MUCH more go, even versus the petrol cruisers (I have been in a 5.7l V8 Lexus version of the 200 in the Middle East too) because of the low-down torque. The more sophisticated suspension makes for better ride & handling on-road.
The Touareg is probably the better long-distance cruiser at high speed but the road quality and traffic volume (and plod/cameras
) means high speed motoring doesn't really exist in the UK, so the Touareg's performance benefits are moot.
Off-road, the Touareg suffers badly versus a cruiser - it lifts a wheel very easily with the independent suspension all round and the road tyres (obviously you're limited because it requires high speed rating tyres for insurance purposes and I think 18" are the smallest it will take) pretty vulnerable to damage. The traction control doesn't overcome these issues successfully. It handles bad dirt roads ok but it bumps and scrapes when the going gets tough because of limited articulation and clearance - for sure it would end in damage eventually. It's simply not an off-road vehicle - just like the X5, ML, Q7 etc. It can do a job off-road but is certainly not the best tool for the job.
The Touareg is a bit smaller inside than a 100 series and doesn't have the 3rd row of seats so will carry 5 people max. It also has air suspension, so the same vulnerability as AHC. Horses for courses at the end of the day. If you're never going to go off the beaten track and can use the on-road advantages, then the Touareg is great. But longevity may be an issue. The LC is a better all-rounder, making up off-road for what it loses by comparison on-road. Reliability is maybe less important in the UK with the AA just a phone-call away but in Africa breaking down is no joke.
As a final comment, my dad is now thinking of getting a 200 series, despite zero mechanical issues with the VW.
Nathan, if you're thinking of going the high-performance diesel route, test drive a Q7 V12 TDI
Cheers,