From a post on www.slantsix.org where I also frequent and posted the same question. I will order the nitrile seal since it will be riding on the speedi-sleeve which is: "SKF SPEEDI-SLEEVE is thin-walled, 0,28 mm (0,011 in), and made of high quality stainless steel. The contact surface is wear-resistant and machined to minimize directionality (0° ±0,05) with a finish of Ra 0,25 to 0,5 μm (10 to 20 μin), depending on size."
"If your crank has the tiny, microscopic angled serrations on it at the seal surface, these will wear out a nitrile seal faster than a leather one. When I had my crank polished, I had them polish out these groves, and then used the nitrile seal. These tiny groves are angled in a way to push the oil back into the engine. But they function as a file against the plastic. Once good plastic seals were developed, they stopped applying this technology. If yours does not have the groves, then the plastic is good.
Check it out and see if they are there. If so, stick with the leather. It will work fine.
Sam"