This kind of depends on how much you sweat. I have a moderate case of hyperhidrosis, and couldn't have chosen a more incompatible passion!

I started out with a Metolius chalk ball filled with Super Chalk, and it was horrible. Here's why: the sock's main purpose is to give conservative doses of chalk, which proved too stingy for my needs. On top of that, it also acted like a sifter, coating my hands with only the finest particles of chalk and "secret additive" (thereby negating the benefit of mixed textures). This fine dust was quick to absorb moisture, but also turned into a slippery paste when exposed to too much.

After that, I tried a block of Frank Endo chalk, and crumbled it straight into my chalk bag. This was grittier, giving me a better grip, and I liked the combination of dunking in the powder, and pinching the pebble sized bits (for finger contact and meditation).

Seeing that I still had some chalk in the abandoned sock, I later emptied it into my chalk bag, and discovered that it added some drying power to the grittier stuff.

Feeling experimental, I then bought a bag of loose Super Chalk and tried using that alone. This turned out much better than sifting it through the sock, but I still got a coating of slurry from the finer particles (albeit a lighter coating). I now mix about a 1:3 to 1:4 ratio of Super Chalk to Frank Endo. This mixture dries better than uncut Endo and lasts longer between dunks.

Now, regarding what I think the additive to Super Chalk is, my guess would be starch. For normal amounts of sweat, this seems like a good choice. Corn starch has great absorbing power, and it's non-Newtonian properties (do a google search for for "ooblech") would seem beneficial for climbing.

For me, however, the non-Newtonian properties only came into effect when I rubbed my hands together vigorously to get the stuff off (THEN it would crumble back into dust). This is because the place where the slime forms is also where my hands sweat most: the edge from the base of the pinky, down to the wrist, and back up to the side of my thumb. This area get's very little contact compared to my fingers, so there isn't constant force/pressure keeping the slurry in a solid state. Consequently, when I do have to use those sufaces (mantling, hand-smears, slopers) I need to clean off (t-shirt, forehead, leg hairs, etc), redunk, grab, and move before the fresh powder turns to mud.

Of course, that's all under the assumption that the additive is starch (Pending experiment: Adding some corn starch to Frank Endo chalk and see how it performs compared to Endo + Super Chalk).

So, to summarize: Coarser stuff will not turn to slime, give better granular traction (if needed), but will also come off your hands quicker (more dunking). Finer stuff (including starch) dries faster and stays on longer, but turns to paste if sweating is excessive.

That's probably more information than you'll ever need to know about chalk, but that's coming from someone who's alway covered in it (hence the handle "blackboard")

Ok, I'm going to stop typing,...now.