rythmik driver is metallic, and GR driver is paper. I spoke to both of them, and GR driver can be crossed slightly higher than the metallic one, and supposed to be a bit more dynamic. Plus its a bit more cheaper. I'd say, you won't go wrong with either of them, if you plan on getting the STs.
Boy, i wish i could afford salk speakers! The bamboo ones Jim posted on audiocircle were breathtakingly stunning! Am yet to come across a better speaker builder than Jim! I think Jim has built a rythmik sub to go with STs for a customer of his. Ask him, and you might be able to match the sub with the speaker!