Hello,
Exciting times from making a decision to change the camper to a different model, and in your case to make the decision to get a VW GC 600 or 680.
To decide whether the GC 600 or GC 680 is better for you is difficult. I suggest to ensure that you have a good look at the two vehicles and, if possible, make a longer test drive.
We have the GC 600 for a while now and also will only be used by two people. What I like is the length of the GC 600 which is really a bonus either by driving winding roads (and we have enough of those in Ireland), manoeuvre on a campsite or find a place to park the camper.
Because neither my wife and I are very tall we have no problems to sleep in the GC 600, actually we sleep rather well. Have I said that, if my wife (sleeps close to door) needs to get up at night, I have also to get up.
What I miss on the GC 600 is the storage space. It is ok for cloth, but it is not ok for kitchen utensils and food.
I can imaging that the GC 680 beds could be even more comfortable, because they are wider and longer.
The weight of the camper is a different story. 3.5t is not a lot. I believe it doesn't matter if you get a GC 600 or a GC 680 if you fully spec it up and load it for a 3x weeks journey you easily will be over 3.5t. Before we left for our holidays I went to a weighbridge. The GC 600 was almost prepared for holiday. The weight was 3,350 kg. What was missing was me, my wife, the dog and food. Meaning we were to heavy. My wifes demand to bring the folding ebikes with us is a nightmare.
I asked my VW dealer and VW Ireland if it was possible to change to 3.88t or 4.1t. 3.88t should be only a paper exercise. But the feedback I've got was very negative.
But thinking about it, I would like to stay with 3.5t limit. Not for the driving license, not for the resale value, but simply that the nicest roads in Ireland but mainly in France are forbidden for vehicles over 3.5t. This really affects the journey (well, at least for us).
What would I do looking back buying our camper? I would put more effort see the GC 680 (in 2021 when we bought our camper there was no GC 680 in Ireland at all). It appeals to me that there is more storage space near the kitchen area. Also the beds might be even more comfortable. What I would have to think about is the harder maneuverability of the GC 680 and, potentially, the weight.
Good luck with your journey and decisions.
Regards,
Eberhard