First, let's take a look at Cato Sicarius and his signature squad.
Sicarius himself has good fighting stats (2/6, 2/5 when bloodied), and features a very powerful Reaction that gives you a resource every time an enemy unit at his planet is destroyed (note: sacrificed is not destroyed). This ability means that you have slightly less need for resource economy from other sources and that having Sicarius present at major battles can provide you with a serious economy boost. Further, these stats and this ability make Sicarius a strong choice for sniping enemy capping units, as you will gain a resource in addition to the command struggle benefits these plays offer.
Fury of Sicarius is a powerful Reaction that lets you kill an enemy unit outright after damaging it with a Space Marine unit. Note that this card does not target the enemy unit in question, so it can't be countered by Communications Relay or Land Raider. However, the enemy does get a chance to shield before you can use Fury of Sicarius, so plan accordingly. This card can be a strong way to deal with enemy heavy hitters, especially Flying ones, but don't hesitate to use it against smaller targets if necessary.
Next, we have Tallassarian Tempest Blade-- one of the best cards in the core set and likely the standard to which all future signature attachments will be compared. Not only does this card give your Warlord +1 attack (very powerful on its own), it grants Armorbane, which allows him to reliably kill units without fear of shielding. This card has an incredible impact on the game, especially if played early.
Sicarius's Chosen is a solid core unit. The Chosen don't have the greatest stats (three resources for a 2/3 with one command icon is pretty "meh"), but they also come with a nasty Reaction that can be used to pick off enemy capping units in the early/midgame or to misdirect heavy hitters from important battles. Overall, they are a solid addition to the Space Marine bag of tricks, though not a critical one.
Finally, we have Cato's Stronghold. Like most signature supports, this card is very good-- it works well with some of the more aggressive Space Marine cards, like Daring Assault Squad, Honored Librarian, or even Sicarius himself (especially when equipped with Tallassarian Tempest Blade). Unfortunately, there is currently not a way to get this out very consistently, so I wouldn't rely on using it.
This Warlord and signature squad leaves us with a pretty strong base to begin deckbuilding, with some solid core units and great support cards. Moving on from there, we notice that the Space Marine cardpool is somewhat limited when it comes to command struggles. It has only two one- or two-cost units with command icons and lacks a cheap two-icon option. On the other hand, we have several good combat units and great tricks with which to support them-- tricks which only work with Space Marine units.
Based on this, I tend to favor Tau allies over Astra Militarum ones. They offer three main advantages:
1. Better command options. Tau have more 1-drops than Astra Militarum does, and I strongly prefer Vash'ya Trailblazer to Sanctioned Psyker. Further, Vior'la Marksman is objectively better than Ratling Deadeye (same cost/ability but +1 HP).
2. Earth Caste Technician. This card lets you search/filter for attachments, letting you get your crucial Tallassarian Tempest Blade out quicker.
3. Ion Rifle. This is a great card for turning cheap units into relevant ones and medium units into heavy hitters.
While the Astra Militarum has some strong options of its own, like Catachan Outpost or Assault Valkyrie, it doesn't fill the Command weakness as well as Tau do. Further, our Space Marine combat tricks don't work with Assault Valkyrie, Daring Assault Squad outclasses Mordian Hellhound, and Elysian Assault Team is somewhat redundant with Eager Recruit. Thus, I think that Tau are a better choice at this stage.
My typical "allies package" is:
3x Vior'la Marksman (early capping, ranged threat against cheap units, pings warlords who come to snipe him, can be made significantly more threatening with Ion Rifle)
3x Earth Caste Technician (capping, searches/filters for attachments)
3x Vash'ya Trailblazer (excellent capping unit)
3x Ion Rifle (useful attachment and shield)
This set of units helps fill the Space Marine weakness in the command struggle while also giving me a better chance to get Tempest Blade out early and providing a solid attachment (Ion Rifle) that can either be used offensively (as an attachment) or defensively (as a shield). Ion Rifle on Honored Librarian or Deathwing Guard is an especially good combination.
The next step is to select the rest of your Space Marine cards. This is relatively straightforward so I won't explain it all here, but it basically boils down to getting a strong set of cards that don't step on each other's toes too much (for instance, Blood Angels Veterans is a solid card, but is somewhat redundant with Sicarius's Chosen). My current decklist ends up looking like this:
Captain Cato Sicarius (Core Set)
Army Unit (33)
3x 10th Company Scout (Core Set)
2x Daring Assault Squad (Core Set)
3x Eager Recruit (Core Set)
3x Earth Caste Technician (Core Set)
3x Honored Librarian (Core Set)
2x Land Raider (Core Set)
4x Sicarius’s Chosen (Core Set)
3x Tactical Squad Cardinis (Core Set)
3x Vash’ya Trailblazer (Core Set)
1x Veteran Brother Maxos (Core Set)
3x Vior’la Marksman (Core Set)
3x Void Pirate (Core Set)
3x Ion Rifle (Core Set)
3x Promotion (Core Set)
1x Tallassarian Tempest Blade (Core Set)
3x Drop Pod Assault (Core Set)
3x Indomitable (Core Set)
2x The Fury of Sicarius (Core Set)
1x Cato’s Stronghold (Core Set)
1x Fortress-Monastery (Core Set)
Note the 18 1-2 cost capping units and 3 Promotions. I've found that this deck often dominates the command struggle-- far from what we would expect from a faction that normally has this as a weakness! I've also removed some of the "heavier" units like Ultramarines Dreadnought and Deathwing Guard-- while these units can be good, they tend to be expensive and vulnerable to tricks. Having more smaller units and focusing more on card draw (easy to do with Sicarius's extra resources) seems like the way to go.