Now that we have rules for the new companions, I'm doing a comparison.
Guardian Beast
Our tried-and-true aspect from the very start, Guardian Beasts are Familiars who over generations of war have been optimized for combat by mages. A Beast is uplifted with a full set of spells that all work well for the Beast's class, as well as special benefits if we make it at random. However, a Beast can only effectively act within its class, and is likely programmed with hindrances to keep it from attempting the other classes (Samantha can't shoot with any degree of accuracy, Olivia likely has a pacifistic attitude, Gears are uncomfortable outside of a workshop, etc).
Familiar
Has all the same physical properties of an average Guardian Beast, but without the pigeonholing of assigned roles. Can be as specialized or generalized as the mage wants, and therefore can cover a wider range of scenarios depending on the spells as well as allowing for thought-out plans for spell synergy with the mage. It also allows a work around for the "D-rank can only have Gears" restriction as the only drawback for a D-rank Familiar is the loss of one spell slot. However, a Familiar only begins its existence with a single spell, and must gradually build up its options over time. A Familiar is unlikely to outperform a Guardian Beast in its given class unless fully specialized, and even then the Familiar will be lacking in the specific strength that GB can bring.
Iron Defender
The crazy one. Defenders have one less spell slot due to their unnaturalness, but can have one spell related to their function be extremely powerful. They also retain any additional mechanical advantages their original form possessed. Being inorganic, they are immune to asphyxiation, starvation, or fatigue; and can resist a larger range of temperatures without damage. However, they would be weaker against electric-based attacks, they also must work their way up to a larger range of spells, and depending on the original form may have limited stealth ability.
My opinion on the remaining girls is as follows. Kayleigh should go for a Guardian Beast of the Gear since it covers her manatech requirements immediately. I'm still voting for the gliding gerbil, but that's just me.
Missy can convert Zeus into a Familiar. Her Guardian Summon is extremely similar to a Guardian Beast, and once we find out its abilities, we can build our Familiar's spell sheet in a way that compliments and covers for both Missy and her summon.
With Laura...I'll admit, I'm still wanting to do a random Guardian Beast for her. Seeing the bonuses Cat Sith and Olivia received are very enticing, and I originally asked about robot Beasts as a joke, so while it is growing on me, I can't envision it just yet. Might change my mind later, we'll see.
