It really depends on if you want an army that goes well with your current army (Daemons), or an army that is completely different (Tau).
In larger games (
Apoc) you could easily combine your Chaos with Daemons to make a really cool looking force. Wouldn't work so well with Chaos and Tau...
However, since 5ed favors
CC so much more than shooting, I think Tau would give you more of a tactical challenge than Daemons