I'd probably go with two squads of three, since you're very fast, ignore terrain and can re-roll charge distances; anything you need six Wraiths for can probably be charged by both units without too much difficulty, and since you're Fearless you don't need bigger squads for morale, and since you're purely combat with no shooting there's no risk of shooting with one squad and leaving the other out of range. In return you can engage more targets, and you're even harder to kill (you can't get all six wiped out by some kind of insane dakka unit, which are becoming more and more common). For fast melee units, I'd advocate always bringing a maximised number of minimised squads. It's usually applicable to all other kinds of units to - fill out your FOC before you fill out your squads. It's almost always better to do so unless you're using a lot of unit-based buffs.
Last edited by MidnightSun; 01-07-16 at 06:23 PM.