I don't know Bushido or Ronin, but I have both Test of Honour and Clash of Katanas as well as Zenit's Kensei and Torii rules. I'm happy to play all of these.
Test of Honour and Torii typically have about a dozen figures per side – i.e., very small skirmisher combats centred on a hero with some companions and followers. Clash of Katanas and Kensei have about a half-dozen units of typically 6-10 figures per unit – i.e., more "military" skirmish games of 25-100 figures per side.
The other key difference is that both Test of Honour and Clash of Katanas are historical while Kensei and Torii have Japanese fantasy elements (e.g., Oni, etc.). The clans for these rules are not historic but based on Zenit's fictional land of Hymukai.
I'd happily play all of them. My wife enjoys Test of Honour so that's reason enough to play it, but I also like the quests, skills and activation mechanism. However, I'd use Clash of Katanas for small, historical battles. I see the two sets of rules as complementary.
The Zenit rules are for when I want to include a fantasy element. The Kensei rules are only avaiable as a hard copy in Spanish but you can get a free pdf version from Zenit. The Torii rules are only available in their Torii army boxes with 10 figures.
