As always, the biggest factor influencing your choices should be the type of enemy you are facing and the tactics you would like to use.
Here are some general guidelines for choosing between the above units.
White Lions of Chrace
This unit has the highest strength of the three (str 6 with their double handed axes)
Importantly, they are also stubborn (no leadership reduction due to combat resolution modifier)
As a result this unit is best deployed against higher toughness opponents. You can rely on their stubborness for about 3 rounds (unless you get unlucky).
Swormasters of Hoeth
The most skilled with a weapon and the most devastating against lower toughness opponents (due to their 2 attacks).
These troops should be used as a shock elite unit. Give tham a good leader, several ranks and a banner. You want this unit to wade through the enemy quickly and not get tied up in combat for long.
Also ideal against enemies with a weapon skill of 5. Because, unlike your other specialist units, the swordmasters with only need a 3 to hit.
Phoenix Guard
The perfect unit against lower leadership opponents or low skill high toughness enemies. This unit should survive longer than your others with it's 4+ ward save. Make sure you deploy them in high numbers, at least 20 in a unit, preferrably 30.
You will quickly mop up lower leadership units by makng them rout using your Fear factor.
Don't deploy these against tougher opponents, you will need them if the battle is long an drawn out to mop up the enemy and walk ahead soaking up the missile fire from stragglers.
