While in some areas of the media/car community diesels are somewhat 'demonised' now, unless you regularly need to drive into cities well into the future we'd say it's a wise swap. You should be getting better fuel economy from a TDI Touareg than a 4Motion TSI Tiguan in most circumstances, so that benefits CO2 emissions, while the newer diesel such as the Touareg are a high enough emissions standard (greatly reduced NOx/particulates) to have no restrictions on them. In 5-10 years time? We can't say for sure that stricter regulations won't mean potential charges required to enter cities.