I'm building up an SP-01 SAO 9maj gun right now because I cant/don't want to afford a TSO, and I love modding things. I had an S2 and sold it because I just like the SAO triggers.
What I REALLY wanted to do was find a TS (not the orange), and build on that, but they don't make those anymore, just the TSOs. Some folks do not like the thumb rest, optic mount and magwell that comes with the "O", so... to me that kind of negates the "O" factor. I figure with a starting price of 600, I can get pretty close to what I want for another 500 or so. Guess we will see.
One other thing I just noticed is that the TSO sear cage is sprung, pretty neat feature.