Some Fimo, but mostly Kato. Fimo has gotten too soft for most of my work... Actually, I'm having trouble with the softness of Kato lately too. I've had to leach most of the clay I use...
I use both Fimo and Premo Sculpey. I also like to mix the two. I'm addicted to Fimo's transparent. I like to add cinnamon or red pepper to it to get great earthy blends.
Fimo - I do a lot of cane work, and I've found it to be the strongest but not too flexible. Sculpy is much too soft - it flops over, and takes on fingerprints like nobody's business.
I like both, but depending on what I am making!! I use FIMO for stronger and big sculptures and Sculpey (translucent) for small ones...like the end of a mermaid tail!!
It depends mostly what I'm using it for. I use Fimo for things like plates and such, because it's harder and keeps it's shape better. But for everything else, I usually use Sculpey, because it's easier for me to work with.