The semi-slav is probably the better in absolute terms as far as best winning/drawing chances but it is very complex with many subitles. The Tarrasch is easier to play, requires less study but gives you greater problems if White knows what he is doing.
I've been dabbling a bit with these systems, and I like both about the same.
Anybody have a strong preference? What are the advantages/disadvantages to one vs. the other?