To back up Kates point. Down wins hands down in my opinion (-sorry about the pun ).
As a family (4) we do most of our camping in Scotland. We have 14 sleeping bags in the house only one of which is synthetic, and that only gets used for Scout camp, where I cannot trust the other boys to respect my boys gear . If in doubt I use a SB cover to protect a Down bag from damp - but only rarely.
Try a wanted Ad on OM and see what folks have to offer - you may do very well out of it - there are some compulsive gear hoarders around .
The point I was trying to make was that:- 'Joe Bloggs' now knows that 'wild camping' is legal in Scotland. He hasn't read the access rules, doesn't think or care about the mess he leaves behind, he just thinks he can camp for free in a nice spot by the road and he is legally allowed to do so. I'd love to think you could educate 'Joe' into not littering etc. but the evidence for that is not good. The kids in Primary school get the 'no litter' message fed to them repeatedly - guess what? - a certain proportion of them don't think it applies to them, and litter the school and everywhere else. I wish I knew the answer.
I love that Scotland has 'open access' to it's countryside etc, however, I question if this would be appropriate for England/Wales with it's much higher population density. Maybe the rules need to be different.
Glen Etive is a case in point. I camped there (from the car) many years ago before it became 'legal'. So did many others, but they were mostly climbers and walkers and took care and cleared up after themselves. Now it is legal many other less aware people are abusing the place and it is being trashed. I would not camp there again - for me it has been ruined by 'the right to camp'