For the most part I use Premo, although I still have a few bits of old formula Kato. I make pretty much anything out of both of them, including beads, buttons, cabochons and clasps.
I loved the old formula Kato, the firmness of it was perfect for canes and keeping the definition, plus the colours seemed more intense and stayed truer after curing. I'm not as keen on the new formula Kato as it is quite soft compared with the old formula.
Now I mainly use Premo, which is fantastic as it has an artist's palette of colours, making colour mixing options amazing. It is less labour intensive than old formula Kato, but is still strong and durable.