What a lovely dilemma to have
Llanwenog - great for dyeing, shortish staple, for carding and spinning woollen, makes a bouncy yarn.
Zwartbles - not so good for dyeing as it's black to start with! May have a longer staple and therefore be both cardable and combable, or you could flick and spin from the fold. IME, not such a bouncy yarn, you may want to blend it and/or ply it with something else.
AFAIK, neither has any particular tendency to felt.