I am no user of either. But based on the specs and the pics I saw on your links, I'd be a wee bit inclined to favor the Swan 6.1. It has a 1db higher sensitivity than the Onix which OTH has a lower impedance rating that will extract more current form your amp and, thus, make it work harder and hotter. And it looks gorgeous to me. But that's just my preference. I really would suggest you audition these babies with the amp you have in mind. There's no subsitute to a real listening comparison on the same amp.
