Hi Cousinbillyl,
You say you already have an Airy, so you're familiar with its sound? If you can say what direction you'd like to move it would help in choosing a second cartridge. It would also help to know what your second arm will be.
I second Larry's comparison of the RSP and Airy, except that calling the Airy "detailed" might leave the impression that it's a detail freak's cartridge, which of course it isn't. Rather than "more detailed" than the RSP, I'd call the ZYX slightly more lifelike or present. The RSP gently rounds leading edges, gently smooths decays. This accounts for its famed lushness. If a very sophisticated romantic's sound is what you're after, the RSP will bring you there. The Airy doesn't do that, but I don't know if this makes it any less musical. To my ears the Airy and RSP are about equally so. (Then again, Larry is a very sophisticated romantic and I'm not!)
I admire Larry's idea of waiting until you've heard the Airy/Teres/Schroeder combo before choosing a second cartridge. Your second step will more likely be in the right direction if you know where your first step has you starting off from.