Benjamin--I do find the Ion quite a bit more breathable than my old Ether [pre-HP version]. I have done comfortable jogging in 40-60F weather without overheating, and that's with the jacket zipped up 75-90%. The amount of moisture inside was much less than similar runs with the Wisp, slightly less than with a Montane Featherlite. Aside from that, I think the Ion fabric is more comfy next-to-skin. It doesn't stick as much, drapes better, and makes for a less humid microclimate when you're just hanging out.
I'll close by reiterating that I have no experience with HP versions of the GoLite fabrics, though I've heard they're better. If you like the Ion fit, I think it's a great option.