So you’ve decided to get started with Infinity and want to pick a faction to play. This can be a daunting task, as there a seven factions with a large variety of models to choose from. This is by no means an “if this, then pick this army” guide, but might offer some useful advice.
Each faction in Infinity has a clear visual style. While e.g. Aleph has a futuristic look with its cyborg infantry and clean, white official paint scheme, Ariadna is much closer to current military forces. If you want aliens, Combined Army or Tohaa might be your thing. If you like Ninjas, go Yu Jing. I like blue and Mecha, so PanOceania was a natural choice for me. USAriadna is a close second for me, as I dig the more contemporary look.
Each faction has its own play style, due to different unit characteristics and special rules. The Tactics articles on 1d4chan give a good idea of how the different factions play. Ariadna e.g. focuses on cheap low-tech infantry (but with camouflage and often well armored with armor-piercing ammo), Haqqislam has a high WIP (willpower) rating that helps their hackers, doctors, and engineers. PanOceania has relatively expensive units with lower WIP but good BS (ballistic skill, the value used for shooting), so you will often see fewer units on the table.
Sometimes, a single unit that you really like can make you go for a certain faction. Browse through the online store to see what is available. To me, it was the Dragoe that first pulled me into PanO.
Besides the “vanilla” lists, each faction (with the exception of Tohaa at this point) has access to Sectorials. The main benefit of playing a Sectorial is that you get access to fireteams, which offer several benefits. Unit choice is different from playing the vanilla list – some units will be available in different numbers, some units not available to vanilla may be there, and some units are not available at all. This is something worth investigating to prevent you from buying lots of models that are not playable or suboptimal for the sectorial of your choice.
Not all units that are available in the game have a model/miniature available for them, and certainly not in all possible weapon configurations. USAriadna e.g. was hampered for some time by a lack of models, though this has been redeemed now. You might also notice that older models are somewhat smaller and sculpted in a different style, which may or may not be an issue for you. So be sure to check whether the faction/sectorial you want to play actually has most of the units available as model, otherwise you might have to proxy or convert a lot of models. Keep in mind though that Infinity has no WYSIWYG requirement. If you announce at the start of the game that a certain model has a different weapon than what the miniature carries, your opponent should be fine with that.