I'd think that the fully synthetic one would be the better of the two, though if VAG had come out with a better fluid then it would be even better. But as you say most people do not ever change the gearbox oil - I don't, I've only ever changed the fluid in a FWD coupler - but that was meant to be done at a certain mileage or when it had become sort of caramely. That car, a VX Cav GSI, did tend to use a transmittion fluid a bit like power steering fluid in the gearbox, so, if VAG ever changed the "stuff" for your specific model of gearbox to that sort of stuff then it might make the box feel better - mainly at start-up in cold weather, until then, do as you seem to be doing - ie just stay thinking about it!