Light Armor vs Heavy Armor. 1st Companion Tank vs 1st Companion Healer. Very easy to tell why one would level "easier" than another.

Qyzen Fess gets just as much melee damage/crit from strength as he does from aim. Only his 2 aoes don't benefit from strength. If you're wanting him to not use those anyways, then feel free to make him look like a jedi and his single target abilities will scale the same. Qyzen never has auto-switching issues for me so long as me or him initiates aggro. As balance I have negligible problems and little downtime by using Ctrl+1 to send him after something and he's usually there by the time I'm finished casting Disturbance on another and he usually gets the peripheral aggro from all the other mobs. Techblades are sold by vendors at the beginning of every world and sometimes twice on the same world. Keeping his weapon up to date is relatively easy. @lvl 20, I've been leaving him in Combat Stance since I single target everything down anyways. Even on elites, between Conq Strike (bonus threat) and Taunt, I don't rip aggro until sub 30%. The guard stance mathmatically only gives about 5% damage redux, which hardly makes up for 25% damage bonus and 25% CD redux. Him doing ~35% more damage will mitigate more damage than 5% from guard stance.