I've pretty much decided to buy one of those two boxes. Which one seems "more complete" with little need for additional purchases ? Which one may perform better against Haqqislam, Yu Jing and Scots (he might buy Aleph in the near future too) ? I'm posting this on both forums to hear you out. I can't find any comprehensive fluff on Onyx and Toha (just some podcasts) - can you give me a link ? I would like to read about both armies.
The Tohaa are an advanced alien civilization currently waging an open war against the EI and its Combined Army. Guardians of one of the great treasures of the universe, the Tohaa have withstood the EI’s onslaught longer than any other race in this galaxy. The Tohaa are masters of biotechnology, capable of building terrible viral weapons and impervious symbiont armors, but also of altering other species, granting them more intelligence so they can fight by their side. The Tohaa need allies, and Humanity seems fitting for the first line of the fight against the Combined Army. from Infinity Official website.
Both armies are completely different playstyles, what is your playstyle? Both are 300 points, your always going to need more models. This thinking doesn't apply in Infinity, player skill trumps list every time. The fluff is in the books, and per CB request to the community it doesn't get splashed up on the internet. What is awesome is this community honors the companies wishes and doesn't just steal the content and paste it online. @DeadlyOcean has the general fluff. For specific units the online store has a blurb of fluff under each unit. Sorry if my answers arn't very helpful, but if you approach infinity like you would 40k or sigmar you will struggle.
the tohaa box is all useful models, and things that you would usually end up buying anyway if you were going to do tohaa. the onyx box is less so, mostly because onyx is a sectorial for combine, and playing onyx, you will prolly wanna branch into combine, and then you might realize vanilla has better options than onyx, and then on from there. tohaa has the least overall options for models (aside from the new merc factions) and is therefore the easiest to "complete". honestly, id suggest the same thing i suggest to everyone, try a few games with proxied models before you buy into it, to see if you even like what you get. that being said, both are great deals, and if it ends up that you don't like the army you get, you can just use them as proxies for whatever army you do like.
Buy both? I do think that the Tohaa 300 point box is a better deal for what you get, but it really does boil down to which ones speak to you more and fit your play style.