I think you might also need to consider the HQ your army is taking:
Avatar - Strong CC threat and fearless bubble, great for footdar list that like to move. Based on that, I would give Dire Avengers the edge, since they are better in a foot-slogging role, with their enhanced range, better armor, and access to a potential Invul Save
Farseer - Great force multiplier HQ, especially with Divination, as he can add to the unit durability and firepower. Given that the buffs are the same no matter the unit size, I would give Guardians the edge with a Farseer, since they can potentially benefit a squad double the size of a max DA unit with all of the Farseer buffs.
Spiritseer - Again, another force-multiplier unit, albeit similar to the Warlock that Guardians can take naturally. That said, he has higher leadership and access to more than one power, so much more reliable in casting and also in getting useful psychic abilities. Given that many of the powers from Runes of Battle either buff friendly CC or debuff enemy CC, Avengers can take advantage of this more since they are naturally better at CC than guardians (leadership, armor, counterattack, Exarch, etc.), though still not a good dedicated melee unit. As for the other Runes of Battle powers, I would argue that they provide greater advantage to Dire Avengers (i.e. turning DA armor into MEQs is better than turning Guardian armor into Storm Trooper, running further is even better for the DAs since they can either shoot and hide better or run and then shoot at even longer range, etc.). Overall, while really designed to support melee troops and/or Wraith units, I think a Spiritseer HQ lends itself to taking DA over Guardians.
Autarch - More of a support character, either in melee (i.e. bike, Mantle, special CC weapons) or on the backline (man the Icarus with high BS while modifying resrves beg=hind a fortification). In this sense, I think the advantage leans towards Guardians, who are often either camping on a backfield objective with Heavy Weapons and a ADL or are meching forward in a Wave Serpent and need some melee counter-attack/distraction support when they jump out at Rapid Fire range to shoot their catapults.
Not going into Special Characters, but obviously they can greatly impact the choice of DAs vs Guardians as primary Troops when taking into account the abilities/rules they grant to their squad, the WL traits they bring, and the impact on maneuver/shooting/combat they can have (i.e. imagine Karandras jumping out of a WS with a squad of Guardians and a Warlock.... potential strong cover save in the open due to Stealth/Shrouded (and brutal one in actual cover), great at taking down monsters, unshakeable morale, ability to tank enemy shooting with Karandras, and a scary good melee capability (something you don't normally associate with Guardians) due to big Scorpion Himself... he can basically ace an entire MEQ or TEQ squad single-handedly, especially if they were soften up by Bladestormin' Shurikan catapults beforehand.