Signum seems more like a skilled sword bitch that uses strength over finesse, cutting through her obstacles and parrying attacks. Touka is more about the delicate finesse (so as to balance with Karura), weaving her sword into her opponent and dodging attacks with grace.
tl;dr - Kararu == Signum, Fate == Touka.
Also, lol, both Fate and Signum can fly and have ranged attacks (but probably wouldn't in a fair fight).
Left with the final option, I'm not sure if I can make the decision of Fate/Signum pairing being hotter then Kararu/Touka pairing. Both are pretty awesome so I propose a foursome.