I'm a tad biased but would go for the Zwartbles tup, if for no other reason than if your daughter likes black sheep, she may be disappointed to get all white lambs off her black ewes...
We put a Berrichon on 3 Welsh/Zwartbles ewes (all black with some white markings) last year and ended up with 3 pure white lambs, 1 with Zwartbles markings and one that looks like a goat - I can't even begin to describe the colours (my OH is horrified by him

)
Another benefit to the Zwartbles tup is that the lambs tend to be pretty narrow and easy to birth, but grow well - and Zwartbles meat (even the cross breds) is lovely
