In what way is kosher chicken different from brined chicken in terms of salt absorbtion?

I explained several of the differences in my answer to Brining a kosher bird and also discussed some issues relating to salt consistency in a much earlier answer to Chicken comes out salty... occasionally.

To make a long story short, kashering is a long process with many steps, but the part you're concerned with is similar to the "dry brining" technique some people are fond of using for Thanksgiving turkeys. The meat is salted directly - no water is used - and it is left to rest for a much shorter period, about 1 hour, before finally being rinsed and packed.

Kosher meat isn't "juicy" like brined meat at all; in fact, all other things being equal, it will come out much drier and tougher than unkosher meat, because in the process of drawing out blood (the reason for salting in kashering), a good deal of moisture is drawn out as well. Brining adds moisture to the tune of about 10%; kashering takes moisture away by a similar amount.

Aside from drying out, the only culinary differences you'll find with kosher birds (or other meat) are (a) less blood, uric acid, and other "undesirable" components, and (b) they are naturally saltier than unkosher birds. However, because the kashering process has nothing to do with flavouring, you can't expect any kind of consistency, and it's not uncommon to find that some parts are much saltier than other parts coming from the same bird.

Kosher birds are going to absorb roughly the same amount of salt and water from a brine. The difference is that because they've already been salted, you run the risk of over-brining to the point of being inedibly salty. That's the whole story.

If you want flavourful and moist then get a regular bird and brine it. If you care more about flavour and want to save some time (at a significant cost premium) then go ahead and use a kosher bird, no brine. You can brine a kosher bird (see first link in this answer) but you're going to have to do a little experimenting to get it right - and if you're going to brine anyway, then why bother spending the extra money on kosher meat?

Don't pay any mind to the oft-repeated claims that kosher birds are "pre-brined" or similar nonsense. The inherent saltiness of a kosher bird is significant but also incidental and thus inherently unreliable.

_{(Please also note: Assuming this is a follow-up to your previous question, none of this is going to make the slightest bit of difference if you're just going to plop the chicken parts into a pot of boiling or poaching water. If you're making broth, then you flavour the broth, not the meat.)}

First, a clarification: "Kosher salt", as a product you can buy in a supermarket, is simply pure crystalline sodium chloride, with no iodine and no anti-clumping agents. Crystals tend to be larger in size than pickling salt or table salt, but usually smaller than evaporated sea salt. Kosher salt gets its name because it is the type used to make a kosher bird (or other kosher meat).

With this definition in mind: the process for making a "kosher chicken" is a complex list of rituals that cover everything from coop to soup, but salt comes into the picture because of the directive against the consumption of blood (Lev. 7:26-27; Lev. 17:10-14). A ritual soaking and salting draws out the blood -- but that means the bird is effectively already partially pre-brined when you buy it in the store. The one problem is that the amount of salt remaining in any given brand of kosher bird may be different from other brands, making a supplemental brining by you difficult to calibrate.

Personally, I don't bother brining kosher birds, even though I do brine non-kosher birds. I suggest you try a brand and see how it comes out.

The term Kosher refers to the Jewish law regarding butchering animals, preparing and handling their meat - not only to the salt. The meat of a Kosher chicken will likely react to a brine in the same way as a non-Kosher chicken of similar size.