I have used a Transit AWD diésel for my trips through Africa, parts of Europe and Latin America. Both vans you mentioned have their advantages and disadvantages. Both 4wd systems are rare. Quigley is unknown outside North America, the Sprinter system is only available in North America and Europe. No support elsewhere. The Transit gas engine doesn't have problems with 3rd world fuel and at altitude. The Sprinter diesel like any modern diesel has problems at extreme altitude (even with low sulphur diesel) and with high sulphur diesel. Imy experience low sulphur diesel with less than 50ppm sulphur (not Ultra Low Sulphur with less than 10ppm) is getting easier to find, ULSD is very rare. Both vans, even a stock Ford AWD are capable enough for RTW. The choice really depends on your destination. For the high Andes and the Asian high mountain areas I personally would still prefer a gas engine but I know a lot of travelers with modern diesels. Unfortunately most (but not all) of them had problems with their emission system sooner or later. But all of them could solve them and continue their trip.
