gunslinger is useless because guns are weak.
swordmaster is helpful against mobs, but bosses are a different story. there's only so much damage they can take before resuming to their defensive stances, so it doesn't matter if swordmaster can give you new ways to attacks, since in the end you'll deal the same damage in the same timeframe while not having the advantages from other styles.
royalguard is the best style, but it's the hardest one to use. trickster is the safest bet.
there are 2 additional styles later on, but they eat up the DT gauge. one is better than swordmaster and gunslinger, the other is only better than gunslinger.