Thanks Pascal. If the return on each trade was adjusted for the difference in volatility between SPY and IWM (multiply the return of each SPY trade by 1.18 for example), I wonder what the comparison would look like.
Regardless, I can see why you may not see a SPY Robot as worth doing. If someone wants a lower beta version of IWM, they can simply reduce their position size.
Another idea: junk bonds (eg. JNK, HYG, etc.). They seem to trend quite well and could potentially offer the highest volatility-adjusted returns - and the correlation with equities is relatively low.
I would be interested to see a robot that takes trades in the area of commodities offering the most potential reward-to-risk - so, for example, it would trade XLE, GDX, etc. depending on which one is offering the highest potential reward-to-risk as determined by the robot's calculations. To me, this is preferable over simply a robot that trades one area of the commodities sector.