I think there is absolutly no difference between these two in real performance, definately not in tasks like OS startup, working with office, browsing web, copying files, running all sorts of client-type applications, and overall response of system. Tasks that utilize big portion of CPU could mean a very, very narrow win for Z530, but there is very few of them.
Funny thing is that the worst CPU killer of netbooks nowadays is flash (ppl dont buy netbooks for news games, 3d modelling or encoding videos, I presume). Some flash videos are slightly choppy in fullscreen. Thats not the case with youtube, but for instance videos at pitchfork.tv or vimeo.com etc - but they are still watchebale. Maybe z530 would improve that. Lets say it would add some 1-2 fps:-)
I have mini 12 with z520 and Im happy with it. Z530 would mean unacceptable 3 or more week delay of my order. I just needed 6 cell battery (now available for mini 10 as well) and 80 gig hdd (mini 10 has faster and 160 GB disk as a base)