This may not be much help because a) we've never had South Downs and b) most people big up their own chosen breed - in our case, Ryelands.
We picked Ryelands because they are quite small, docile and we liked the look of them. Since we bought our first ones in 2007, we haven't regretted it. They are smallish, docile, friendly (in the main), easily bribed by food, have great feet (onely ever had one lame), are good mums and lovely fleece. Their lamb is delicious. On the down side, they have a lot of fleece so some shearers don't much like them and you do have to watch carefully for fly strike, because of the quantity of fleece (although all sheep do need careful watching in the fly season).
I suspect that South Downs have similar characteristics to the Ryeland. My advice would be to pick the one YOU love most - getting up at 2am in the cold and dark at lambing time is less of a chore if you love the sheep you have.