Ok, the DO look similar in this picture but there is a huge difference between the two in real life. SO is a duochrome with a really warm orangey-brown base and Henna is just a straight up antiqued gold. On many people it may not make a difference, but on me (pale, neautral-cool) it means that Henna is totally wearable, while SO makes me look rabid. If you have skin with any kind of color though, I can guess that the end result of the 2 would look similar, except that I am pretty sure that you can manipulate SO to become a spectrum of colors (depending on the base color) while Henna will always look pretty much the same.
Not sure if that actually helps anyone or if it was me just getting on my soapbox....
I just came across something on Karlasugar.net that made me realize I don't necessarily need SO. I forgot I have MAC's Henna e/s and it compares a lot to SO.