I bought a few of the SD1s a while ago when Cold Steel had a holiday sale and ended up giving all but one away - they are just too big to carry under most circumstances (but make great stocking stuffers!). They didn't offer the SD2 at the time, but that seems like a more reasonable size. The material they are made of isn't very dense, so you don't give up much in the way of mass by going with the smaller one. Plus the pointy end seems as if it would be more effective since it weighs so little.
In the end, I don't think it makes much difference which pocket stick/Yawara/koppo you choose. The keys to effectiveness are accessibility and training.