6-oxo is more of a suicide inhibitor.
As far as which works better nobody knows as there hasn't been any clinical studies (in terms of the medical community) on 6-oxo so its almost impossible to say which one is better. I would think Arimidex is more potent though because it seems to suppress plasma estrogen more than 6-oxo but how this would effect PCT nobody knows.
If you want to look at it from a purely philosophical standpoint, if 6-oxo was that potent it would be used along with Dex, Letro, and Exemestane in the treatment of breast cancer. It could also show you the fact that you don't need to drive estrogen into the ground to stimulate Testosterone production (as 6-oxo has shown in normal men).