I use 309 from trugrit, mainly because it was the cheapest I could find. It's a bit better than the 321 which most machine shop suppliers sell as well.
I usually put a couple squares of paper in the foil pack, and I've never had any decarb or scale formation. They come out nearly looking like they went in.
One thing to keep in mind with aeb-L is that it loves to warp. Plate quenching helps a fair bit (I use 1" aluminum) but even with that on long and thin blades (fillet knives, chefs knives) they still will warp on occasion. I've had them come out of the oven straight and warp in the liquid nitrogen as well. Generally on anything like that I'll heat treat essentially a drilled and profiled blank, and do all the grinding after HT. Keeping the belt wet with soluble oil coolant will help with heat buildup.
As for cryo it definitely seems to help, but I've tried aeb-L without it a few times, and it's still a pretty good blade.
