"The kind you make yourself" has been mentioned a couple of times in the thread. But, a nice pile of composting organic matter is the Hôtel de Paris in Monte-Carlo for fire ants in some areas, like mine. I know many of you have never encountered fire ants but they are deadly and, at the least, dangerous. I've tried on several occasions to start compost piles and it never fails to become infested with fire ants. Sure, we could put out poisons *but* it'd have to go into the compost and pretty much all the poisons that will work are stated as not being approved for use where edible crops are growing. To me, the OPs question regarding choosing either chicken or horse manure, is a specific question narrowed down to two answers.
I agree that the horse manure may very well be contaminated with pesticides (either insecticides and/or herbicides). Half-lives of some of these chemicals are long-lasting in regards to harming garden crops...they are designed that way to stop weeds (especially broad leaf weeds) from growing for a season or three. And, though their impact on plants may diminish over time there is still contamination present as half-lives never end. Also, lots of horse feed also contains larvacides to reduce fly infestations...that will get passed along to your soil organisms/critters. Naturally, if they were *your* horses and knew the feed that they were eating did not have larvacides or other "treatments" the manure would be some good stuff. Mixed in with hay, though, I would be careful about bringing in weed seeds, too.
Chicken manure...no big worry from herbicides but insecticides might be a problem. Hormones and antibiotics are supposed to be a thing of the past. I, too, would go with chicken manure over horse manure due to the herbicide-use difference.
For me, I'd have to buy bags from the big box stores of the general "mushroom", "cow", etc., composts offered and trust that they're "clean". Otherwise, find a poultry-grower to get some manure from. The days of shoveling cow or horse manure are over for me. I've seen the effects of herbicides in manure but don't think it's just in manure. There has big cases reported of some big name potting mixes/soils that have caused severe problems for gardeners, too. If you can do all your soil amending from homegrown ingredients you are indeed *very* fortunate!
It's been years since we lived in a fairly "clean" world, though, and it doesn't seem to be getting a whole lot better. Disease and pests continue to appear or strengthen and man tries to deal with it...sometimes properly and sometimes improperly. I can you could apply the old saying, "Pick your poison..." to soil amendments.